Optical stack for switchable directional display

ABSTRACT

A privacy display comprises a spatial light modulator and a compensated switchable liquid crystal retarder arranged between first and second polarisers arranged in series with the spatial light modulator. In a privacy mode of operation, on-axis light from the spatial light modulator is directed without loss, whereas off-axis light has reduced luminance. The visibility of the display to off-axis snoopers is reduced by means of luminance reduction over a wide polar field. In a wide angle mode of operation, the switchable liquid crystal retardance is adjusted so that off-axis luminance is substantially unmodified.

TECHNICAL FIELD

This disclosure generally relates to illumination from light modulationdevices, and more specifically relates to switchable optical stacks forproviding control of illumination for use in a display including aprivacy display.

BACKGROUND

Privacy displays provide image visibility to a primary user that istypically in an on-axis position and reduced visibility of image contentto a snooper, that is typically in an off-axis position. A privacyfunction may be provided by micro-louvre optical films that transmitsome light from a display in an on-axis direction with low luminance inoff-axis positions. However such films have high losses for head-onillumination and the micro-louvres may cause Moire artefacts due tobeating with the pixels of the spatial light modulator. The pitch of themicro-louvre may need selection for panel resolution, increasinginventory and cost.

Switchable privacy displays may be provided by control of the off-axisoptical output.

Control may be provided by means of luminance reduction, for example bymeans of switchable backlights for a liquid crystal display (LCD)spatial light modulator. Display backlights in general employ waveguidesand edge emitting sources. Certain imaging directional backlights havethe additional capability of directing the illumination through adisplay panel into viewing windows. An imaging system may be formedbetween multiple sources and the respective window images. One exampleof an imaging directional backlight is an optical valve that may employa folded optical system and hence may also be an example of a foldedimaging directional backlight. Light may propagate substantially withoutloss in one direction through the optical valve whilecounter-propagating light may be extracted by reflection off tiltedfacets as described in U.S. Pat. No. 9,519,153, which is hereinincorporated by reference in its entirety.

Control of off-axis privacy may further be provided by means of contrastreduction, for example by adjusting the liquid crystal bias tilt in anIn-Plane-Switching LCD.

BRIEF SUMMARY

According to a first aspect of the present disclosure there is provideda display device comprising: a spatial light modulator; a displaypolariser arranged on a side of the spatial light modulator; anadditional polariser arranged on the same side of the spatial lightmodulator as the display polariser; and plural retarders arrangedbetween the additional polariser and the display polariser; wherein theplural retarders comprise: a switchable liquid crystal retardercomprising a layer of liquid crystal material arranged between thedisplay polariser and the additional polariser; and at least one passivecompensation retarder.

The plural retarders may be arranged to not affect the luminance oflight passing through the display polariser, the additional polariserand the plural retarders along an axis along a normal to the plane ofthe retarders and/or to reduce the luminance of light passing throughthe display polariser, the additional polariser and the plural retardersalong an axis inclined to a normal to the plane of the retarders.

The at least one passive compensation retarder may be arranged tointroduce no phase shift to polarisation components of light passed bythe one of the display polariser and the additional polariser on theinput side of the plural retarders along an axis along a normal to theplane of the at least one passive compensation retarder and/or tointroduce a phase shift to polarisation components of light passed bythe one of the display polariser and the additional polariser on theinput side of the plural retarders along an axis inclined to a normal tothe plane of the at least one passive compensation retarder.

The switchable liquid crystal retarder may be arranged to introduce nophase shift to polarisation components of light passed by the one of thedisplay polariser and the additional polariser on the input side of theplural retarders along an axis along a normal to the plane of theswitchable liquid crystal retarder and/or to introduce a phase shift topolarisation components of light passed by the one of the displaypolariser and the additional polariser on the input side of the pluralretarders along an axis inclined to a normal to the plane of theswitchable liquid crystal retarder in a switchable state of theswitchable liquid crystal retarder.

Advantageously a switchable privacy display may be provided that may beswitched between a wide angle operating state and a privacy operatingstate. The field of view for privacy operation may be extended incomparison to known arrangements, and lower off-axis luminance levelsmay be achieved, increasing degree of privacy observed by an off-axissnooper. Further, on-axis luminance may be maintained in both wide angleand privacy states of operation for on-axis primary users.

The display polariser and the additional polariser may have electricvector transmission directions that are parallel.

In one alternative, the switchable liquid crystal retarder may comprisetwo surface alignment layers disposed adjacent to the layer of liquidcrystal material and on opposite sides thereof and each arranged toprovide homeotropic alignment in the adjacent liquid crystal material.The layer of liquid crystal material of the switchable liquid crystalretarder may comprise a liquid crystal material with a negativedielectric anisotropy. The layer of liquid crystal material may have aretardance for light of a wavelength of 550 nm in a range from 500 nm to1000 nm, preferably in a range from 600 nm to 900 nm and most preferablyin a range from 700 nm to 850 nm.

Where two surface alignment layers providing homeotropic alignment areprovided, the at least one passive compensation retarder may comprise aretarder having its optical axis perpendicular to the plane of theretarder, the at least one passive retarder having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −900 nm,preferably in a range from −450 nm to −800 nm and most preferably in arange from −500 nm to −725 nm.

Alternatively, where two surface alignment layers providing homeotropicalignment are provided, the at least one passive compensation retardermay comprise a pair of retarders which have optical axes in the plane ofthe retarders that are crossed, each retarder of the pair of retardershaving a retardance for light of a wavelength of 550 nm in a range from300 nm to 800 nm, preferably in a range from 500 nm to 700 nm and mostpreferably in a range from 550 nm to 675 nm. Advantageously, in thiscase increased field of view in wide angle mode of operation may beprovided. Further, zero voltage operation in wide angle mode ofoperation may be provided, reducing power consumption.

In another alternative, the switchable liquid crystal retarder maycomprise two surface alignment layers disposed adjacent to the layer ofliquid crystal material and on opposite sides thereof and each arrangedto provide homogeneous alignment in the adjacent liquid crystalmaterial. Advantageously in comparison to homeotropic alignment onopposite sides of the liquid crystal, increased resilience to thevisibility of flow of liquid crystal material during applied pressuremay be achieved.

The layer of liquid crystal material of the switchable liquid crystalretarder may comprise a liquid crystal material with a positivedielectric anisotropy. The layer of liquid crystal material may have aretardance for light of a wavelength of 550 nm in a range from 500 nm to900 nm, preferably in a range from 600 nm to 850 nm and most preferablyin a range from 700 nm to 800 nm.

Where two surface alignment layers providing homogeneous alignment areprovided, the at least one passive compensation retarder may comprise aretarder having its optical axis perpendicular to the plane of theretarder, the at least one passive retarder having a retardance forlight of a wavelength of 550 nm in a range from −300 nm to −700 nm,preferably in a range from −350 nm to −600 nm and most preferably in arange from −400 nm to −500 nm.

Alternatively, where the two surface alignment layers providinghomogeneous alignment are provided, the at least one passivecompensation retarder may comprise a pair of retarders which haveoptical axes in the plane of the retarders that are crossed, eachretarder of the pair of retarders having a retardance for light of awavelength of 550 nm in a range from 300 nm to 800 nm, preferably in arange from 350 nm to 650 nm and most preferably in a range from 450 nmto 550 nm. Advantageously, in this case increased resilience to thevisibility of flow of liquid crystal material during applied pressuremay be achieved.

In another alternative, the switchable liquid crystal retarder maycomprise two surface alignment layers disposed adjacent to the layer ofliquid crystal material and on opposite sides thereof, one of thesurface alignment layers being arranged to provide homeotropic alignmentin the adjacent liquid crystal material and the other of the surfacealignment layers being arranged to provide homogeneous alignment in theadjacent liquid crystal material.

When the surface alignment layer arranged to provide homogeneousalignment is between the layer of liquid crystal material and thecompensation retarder, the layer of liquid crystal material may have aretardance for light of a wavelength of 550 nm in a range from 700 nm to2000 nm, preferably in a range from 1000 nm to 1500 nm and mostpreferably in a range from 1200 nm to 1500 nm.

When the surface alignment layer arranged to provide homogeneousalignment is between the layer of liquid crystal material and thecompensation retarder, the at least one passive compensation retardermay comprise a retarder having its optical axis perpendicular to theplane of the retarder, the at least one passive retarder having aretardance for light of a wavelength of 550 nm in a range from −400 nmto −1800 nm, preferably in a range from −700 nm to −1500 nm and mostpreferably in a range from −900 nm to −1300 nm.

When the surface alignment layer arranged to provide homogeneousalignment is between the layer of liquid crystal material and thecompensation retarder, the at least one passive compensation retardermay comprise a pair of retarders which have optical axes in the plane ofthe retarders that are crossed, each retarder of the pair of retardershaving a retardance for light of a wavelength of 550 nm in a range from400 nm to 1800 nm, preferably in a range from 700 nm to 1500 nm and mostpreferably in a range from 900 nm to 1300 nm.

When the surface alignment layer arranged to provide homeotropicalignment is between the layer of liquid crystal material and thecompensation retarder, the layer of liquid crystal material may have aretardance for light of a wavelength of 550 nm in a range from 500 nm to1800 nm, preferably in a range from 700 nm to 1500 nm and mostpreferably in a range from 900 nm to 1350 nm.

When the surface alignment layer arranged to provide homeotropicalignment is between the layer of liquid crystal material and thecompensation retarder, the at least one passive compensation retardermay comprise a retarder having its optical axis perpendicular to theplane of the retarder, the at least one passive retarder having aretardance for light of a wavelength of 550 nm in a range from −300 nmto −1600 nm, preferably in a range from −500 nm to −1300 nm and mostpreferably in a range from −700 nm to −1150 nm.

When the surface alignment layer arranged to provide homeotropicalignment is between the layer of liquid crystal material and thecompensation retarder, the at least one passive compensation retardermay comprise a pair of retarders which have optical axes in the plane ofthe retarders that are crossed, each retarder of the pair of retardershaving a retardance for light of a wavelength of 550 nm in a range from400 nm to 1600 nm, preferably in a range from 600 nm to 1400 nm and mostpreferably in a range from 800 nm to 1300 nm. Advantageously, in thiscase increased resilience to the visibility of flow of liquid crystalmaterial during applied pressure may be achieved.

Each alignment layer may have a pretilt having a pretilt direction witha component in the plane of the liquid crystal layer that is parallel oranti-parallel or orthogonal to the electric vector transmissiondirection of the display polariser. Advantageously a display may beprovided with narrow viewing angle in a lateral direction and a wideviewing freedom for display rotation about a horizontal axis. Such adisplay may be comfortable to view for a head-on display user anddifficult to view for an off-axis display user.

The at least one passive retarder may comprise at least two passiveretarders with at least two different orientations of optical axes whichmay have optical axes in the plane of the retarders that are crossed.Field of view for liquid crystal retarders with homogeneous alignment isincreased while providing resilience to the visibility of flow of liquidcrystal material during applied pressure.

The pair of passive retarders may have optical axes that extend at 45°and at 135°, respectively, with respect to an electric vectortransmission direction that is parallel to the electric vectortransmission of the display polariser. The passive retarders may beprovided using stretched films to advantageously achieve low cost andhigh uniformity.

The switchable liquid crystal retarder may be provided between the pairof passive retarders. Advantageously the thickness and complexity of theplural retarders may be reduced.

A transparent electrode and a liquid crystal alignment layer may beformed on a side of each of the pair of passive retarders adjacent theswitchable liquid crystal retarder; and may further comprise first andsecond substrates between which the switchable liquid crystal retarderis provided, the first and second substrates each comprising one of thepair of passive retarders, wherein each of the pair of passive retardershas a retardance for light of a wavelength of 550 nm in a range from 150nm to 800 nm, preferably in a range from 200 nm to 700 nm and mostpreferably in a range from 250 nm to 600 nm.

In one alternative, the at least one passive compensation retarder maycomprise a retarder having an optical axis perpendicular to the plane ofthe retarder. Advantageously the thickness and complexity of the passiveretarder stack may be reduced.

The at least one passive compensation retarder may comprise two passiveretarders having an optical axis perpendicular to the plane of thepassive retarders, and the switchable liquid crystal retarder isprovided between the two passive retarders. Advantageously the thicknessand complexity of the plural retarders may be reduced. High head-onefficiency may be achieved in both wide and privacy modes, a wide fieldof view for wide angle mode and snoopers may be unable to perceive imagedata from a wide range of off-axis viewing locations.

A transparent electrode and a liquid crystal alignment layer may beformed on a side of each of the two passive retarders adjacent theswitchable liquid crystal retarder. First and second substrates betweenwhich the switchable liquid crystal retarder may be provided, the firstand second substrates each comprising one of the two passive retarders.The two passive retarders may have a total retardance for light of awavelength of 550 nm in a range −300 nm to −700 nm, preferably in arange from −350 nm to −600 nm and most preferably in a range from −400nm to −500 nm.

In another alternative, the at least one passive compensation retardermay comprise a retarder having an optical axis with a componentperpendicular to the plane of the retarder and with a component in theplane of the retarder. Advantageously fields of view in wide angle modemay be increased and snoopers may be unable to perceive image data froma wide range of off-axis viewing locations.

The component in the plane of the passive retarder may extend at 0°,with respect to an electric vector transmission direction that isparallel or perpendicular to the electric vector transmission of thedisplay polariser. The at least one passive retarder may furthercomprise a passive retarder having an optical axis perpendicular to theplane of the passive retarder or a pair of passive retarders which haveoptical axes in the plane of the passive retarders that are crossed.

The retardance of the at least one passive compensation retarder may beequal and opposite to the retardance of the switchable liquid crystalretarder.

The switchable liquid crystal retarder may comprise first and secondpretilts; and the at least one passive compensation retarder maycomprise a compensation retarder with first and second pretilts, thefirst pretilt of the compensation retarder being the same as the firstpretilt of the liquid crystal retarder and the second pretilt of thecompensation retarder being the same as the second pretilt of the liquidcrystal retarder.

The switchable liquid crystal retarder may further comprise electrodesarranged to apply a voltage for controlling the layer of liquid crystalmaterial. The electrodes may be on opposite sides of the layer of liquidcrystal material. The display may be switched by control of the liquidcrystal layer, advantageously achieving a switchable privacy display, orother display with reduced off-axis stray light. The display may furthercomprise a control system arranged to control the voltage applied acrossthe electrodes of the at least one switchable liquid crystal retarder.

The electrodes may be patterned to provide at least two pattern regions.Advantageously increased privacy performance may be provided byobscuring image data. The display may be switched between a wide anglemode with no visibility of camouflage structure and a privacy mode withadditional camouflage to provide reduced visibility to an off-axissnooper without substantial visibility of the camouflage pattern to ahead-on user.

The control system may further comprise a means to determine thelocation of a snooper with respect to the display and the control systemis arranged to adjust the voltage applied across the electrodes of theat least one switchable liquid crystal retarder in response to thesnooper location. Advantageously the visibility of an image to adetected snooper may be minimised for a range of snooper locations.

The display device may further comprise at least one further retarderand a further additional polariser, wherein the at least one furtherretarder is arranged between the first-mentioned additional polariserand the further additional polariser. Advantageously off-axis luminancemay be further reduced, reducing the visibility of the image to anoff-axis snooper.

In one alternative for the display device, the spatial light modulatoris a transmissive spatial light modulator arranged to receive outputlight from a backlight. Advantageously the backlight may provide reducedoff-axis luminance in comparison to emissive displays.

The backlight may provide a luminance at polar angles to the normal tothe spatial light modulator greater than 45 degrees that is at most 33%of the luminance along the normal to the spatial light modulator,preferably at most 20% of the luminance along the normal to the spatiallight modulator, and most preferably at most 10% of the luminance alongthe normal to the spatial light modulator. Advantageously the luminancemay be reduced for off-axis snoopers.

The backlight may comprise: an array of light sources; a directionalwaveguide comprising: an input end extending in a lateral directionalong a side of the directional waveguide, the light sources beingdisposed along the input end and arranged to input input light into thewaveguide; and opposed first and second guide surfaces extending acrossthe directional waveguide from the input end for guiding light input atthe input end along the waveguide, the waveguide being arranged todeflect input light guided through the directional waveguide to exitthrough the first guide surface. Advantageously uniform large areaillumination may be provided with high efficiency.

The backlight may further comprise a light turning film and thedirectional waveguide is a collimating waveguide. The collimatingwaveguide may comprise (i) a plurality of elongate lenticular elements;and (ii) a plurality of inclined light extraction features, wherein theplurality of elongate lenticular elements and the plurality of inclinedlight extraction features are oriented to deflect input light guidedthrough the directional waveguide to exit through the first guidesurface. Advantageously a narrow angular output may be provided by thebacklight.

The directional waveguide may be an imaging waveguide arranged to imagethe light sources in the lateral direction so that the output light fromthe light sources is directed into respective optical windows in outputdirections that are distributed in dependence on the input positions ofthe light sources. The imaging waveguide may comprise a reflective endfor reflecting the input light back along the imaging waveguide, whereinthe second guide surface is arranged to deflect the reflected inputlight through the first guide surface as output light, the second guidesurface comprises light extraction features and intermediate regionsbetween the light extraction features, the light extraction featuresbeing oriented to deflect the reflected input light through the firstguide surface as output light and the intermediate regions beingarranged to direct light through the waveguide without extracting it;and the reflective end may have positive optical power in the lateraldirection extending between sides of the waveguide that extend betweenthe first and second guide surfaces. Advantageously a switchabledirectional illumination may be provided that may be switched betweennarrow angle and wide angle illumination.

In one alternative where the spatial light modulator is a transmissivespatial light modulator, the display polariser may be an input displaypolariser arranged on the input side of the spatial light modulatorbetween the backlight and the spatial light modulator, and theadditional polariser is arranged between the input display polariser andthe backlight. Advantageously the efficiency of the display isincreased. The additional polariser may be a reflective polariser.

In this case, the display device may further comprise an outputpolariser arranged on the output side of the spatial light modulator.

In one alternative where the spatial light modulator is a transmissivespatial light modulator, the display polariser may be an outputpolariser arranged on the output side of the spatial light modulator.Advantageously the efficiency of the display is increased.

The display device may further comprise an input polariser arranged onthe input side of the spatial light modulator.

The display device may further comprise a further additional polariserarranged on the input side of the spatial light modulator and at leastone further retarder arranged between the at least one furtheradditional polariser and the input polariser. Advantageously theluminance may be reduced for off-axis snoopers.

In one alternative for the display device, the spatial light modulatormay comprise an emissive spatial light modulator arranged to outputlight. In that case, the display polariser may be an output displaypolariser arranged on the output side of the emissive spatial lightmodulator. Advantageously display thickness may be reduced in comparisonto displays with backlights, and flexible and bendable displays may beconveniently provided.

The display device may comprise at least one further retarder and afurther additional polariser, wherein the at least one further retarderis arranged between the first-mentioned additional polariser and thefurther additional polariser. Advantageously the luminance may bereduced for off-axis snoopers.

The various optional features and alternatives set out above withrespect to the first aspect of the present invention may be appliedtogether in any combination.

According to a second aspect of the present disclosure there is provideda view angle control optical element for application to a display devicecomprising a spatial light modulator and a display polariser arranged ona side of the spatial light modulator, the view angle control opticalelement comprising a control polariser and plural retarders forarrangement between the additional polariser and the display polariseron application of the view angle control optical element to the displaydevice, the plural retarders comprising: a switchable liquid crystalretarder comprising a layer of liquid crystal material; and at least onepassive compensation retarder.

Advantageously, the view angle control optical element may bedistributed as an after-market element and may be attached to displaydevices by display users. The element does not require complexalignment. Moiré beating between the element and the pixels of thedisplay is not present and selection of the component with regards topixel pitch is not required. Inventory cost is reduced.

Alternatively, the view angle control optical element may beconveniently factory fitted into display devices.

The various features and alternatives set out above with respect to thefirst aspect of the present disclosure may similarly be applied to thesecond aspect of the present disclosure.

Embodiments of the present disclosure may be used in a variety ofoptical systems. The embodiments may include or work with a variety ofprojectors, projection systems, optical components, displays,microdisplays, computer systems, processors, self-contained projectorsystems, visual and/or audio-visual systems and electrical and/oroptical devices. Aspects of the present disclosure may be used withpractically any apparatus related to optical and electrical devices,optical systems, presentation systems or any apparatus that may containany type of optical system. Accordingly, embodiments of the presentdisclosure may be employed in optical systems, devices used in visualand/or optical presentations, visual peripherals and so on and in anumber of computing environments.

Before proceeding to the disclosed embodiments in detail, it should beunderstood that the disclosure is not limited in its application orcreation to the details of the particular arrangements shown, becausethe disclosure is capable of other embodiments. Moreover, aspects of thedisclosure may be set forth in different combinations and arrangementsto define embodiments unique in their own right. Also, the terminologyused herein is for the purpose of description and not of limitation.

These and other advantages and features of the present disclosure willbecome apparent to those of ordinary skill in the art upon reading thisdisclosure in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingFIGURES, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1A is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a frontswitchable retarder;

FIG. 1B is a schematic diagram illustrating in front view alignment ofoptical layers in the optical stack of FIG. 1A;

FIG. 1C is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising an emissivespatial light modulator and a switchable compensated retarder arrangedon the output side of the emissive spatial light modulator;

FIG. 1D is a schematic diagram illustrating in side perspective view aview angle control optical element comprising a passive compensationretarder, a switchable liquid crystal retarder and a control polariser;

FIG. 2A is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight, arear switchable compensated retarder, and a transmissive spatial lightmodulator wherein the additional polariser comprises a reflectivepolariser;

FIG. 2B is a schematic diagram illustrating in front view alignment ofoptical layers in the optical stack of FIG. 2A;

FIG. 2C is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight, arear switchable compensated retarder, and a transmissive spatial lightmodulator wherein the additional polariser comprises a dichroicpolariser;

FIG. 3 is a schematic diagram illustrating in side view an arrangementof a compensated switchable liquid crystal retarder;

FIG. 4A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder comprising a negativeC-plate in a wide angle mode of operation;

FIG. 4B is a schematic diagram illustrating a graph of liquid crystaldirector angle against fractional location through the switchable liquidcrystal retarder cell;

FIG. 4C is a schematic diagram illustrating in side view propagation ofoutput light from a spatial light modulator through the optical stack ofFIG. 4A in a wide angle mode of operation;

FIG. 4D is a schematic graph illustrating the variation of outputtransmission with polar direction for the transmitted light rays in FIG.4C;

FIG. 5A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder comprising a negativeC-plate in a privacy mode of operation;

FIG. 5B is a schematic diagram illustrating in side view propagation ofoutput light from a spatial light modulator through the optical stack ofFIG. 5A in a privacy mode of operation;

FIG. 5C is a schematic graph illustrating the variation of outputtransmission with polar direction for the transmitted light rays in FIG.5B;

FIG. 6A is a schematic diagram illustrating in front perspective viewobservation of transmitted output light for a display operating inprivacy mode;

FIG. 6B is a schematic diagram illustrating in front perspective viewsthe appearance of the display of FIGS. 1A-1C operating in privacy mode;

FIG. 6C is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for both entertainment and sharing modes of operation;

FIG. 6D is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin in an entertainment mode of operation;

FIG. 6E is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin in a sharing mode of operation;

FIG. 6F is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for both night-time and day-time modes of operation;

FIG. 6G is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin in a night-time mode of operation;

FIG. 6H is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin in a day-time mode of operation;

FIG. 7A, FIG. 7B, FIG. 7C, and FIG. 7D are schematic diagramsillustrating the variation of output transmission with polar directionfor different drive voltages;

FIG. 8 is a flow chart illustrating control of a privacy display;

FIG. 9A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a wide angle mode ofoperation comprising crossed A-plate passive compensation retarders andhomeotropically aligned switchable liquid crystal retarder;

FIG. 9B is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising crossed A-plate passive compensation retarders andhomeotropically aligned switchable liquid crystal retarder;

FIG. 9C is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 9Ain a wide angle mode of operation;

FIG. 9D is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 9Bin a privacy mode of operation;

FIG. 10A and FIG. 10B are schematic diagrams illustrating in perspectiveside view an arrangement of a switchable compensated retarder in a wideangle mode and a privacy mode of operation respectively comprising ahomogeneously aligned switchable liquid crystal retarder and a passivenegative C-plate retarder;

FIG. 10C is a schematic diagram illustrating a graph of liquid crystaldirector angle against fractional location through the switchable liquidcrystal retarder cell of FIG. 10A for different applied voltages;

FIG. 11A, FIG. 11B, and FIG. 11C are schematic graphs illustrating thevariation of output transmission with polar direction for transmittedlight rays of switchable compensated retarder comprising a homogeneouslyaligned liquid crystal cell and a negative C-plate in a privacy mode andfor two different wide angle mode addressing drive voltagesrespectively;

FIG. 12A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising crossed A-plate passive compensation retarders andhomogeneously aligned switchable liquid crystal retarder;

FIG. 12B, FIG. 12C, and FIG. 12D are schematic graphs illustrating thevariation of output transmission with polar direction for transmittedlight rays of switchable compensated retarder comprising a homogeneouslyaligned liquid crystal cell and crossed A-plates in a privacy mode andwide angle modes for different drive voltages;

FIG. 13A and FIG. 13B are schematic diagrams illustrating in side viewspart of a display comprising a switchable compensated retarder andoptical bonding layers;

FIG. 14 is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising crossed A-plate passive compensation retarders andhomogeneously aligned switchable liquid crystal retarder, furthercomprising a passive rotation retarder;

FIG. 15A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising a homeotropically aligned switchable liquid crystalretarder arranged between first and second C-plate passive compensationretarders;

FIG. 15B and FIG. 15C are schematic graphs illustrating the variation ofoutput transmission with polar direction for transmitted light rays inthe optical stack of FIG. 15A in a wide angle mode and a privacy mode ofoperation respectively;

FIG. 16A is a schematic diagram illustrating in perspective side view adisplay comprising a switchable liquid crystal retarder arranged betweenfirst and second substrates each comprising C-plate passive compensationretarders;

FIG. 16B is a schematic diagram illustrating in side view part of adisplay comprising a switchable liquid crystal retarder arranged betweenfirst and second substrates each comprising C-plate passive compensationretarders;

FIG. 17A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a wide angle mode ofoperation comprising a homogeneously aligned switchable liquid crystalretarder arranged between first and second crossed A-plate passivecompensation retarders;

FIG. 17B and FIG. 17C are schematic graphs illustrating the variation ofoutput transmission with polar direction for transmitted light rays forthe arrangement of FIG. 17A in wide angle and privacy modesrespectively;

FIG. 18A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising a homogeneously and homeotropically alignedswitchable liquid crystal retarder and a passive negative C-plateretarder;

FIG. 18B is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 18Ain a privacy mode of operation;

FIG. 18C is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 18Ain a wide angle mode of operation;

FIG. 19A is a schematic diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable liquid crystalretarder;

FIG. 19B is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 19Afor a first applied voltage;

FIG. 19C is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 19Afor a second applied voltage that is greater than the first appliedvoltage;

FIG. 19D is a schematic diagram illustrating in perspective side view aC-plate arranged between parallel polarisers;

FIG. 19E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG.19D;

FIG. 20A is a schematic diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable liquid crystalretarder arranged between parallel polarisers in series with a C-platearranged between parallel polarisers;

FIG. 20B is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 20Afor a first applied voltage;

FIG. 20C is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 20Afor a second applied voltage that is greater than the first appliedvoltage;

FIG. 21A is a schematic diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable liquid crystalretarder in series with a C-plate compensation retarder wherein thehomogeneously aligned switchable liquid crystal and C-plate compensationretarder are arranged between a single pair of parallel polarisers;

FIG. 21B is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 21Afor a first applied voltage;

FIG. 21C is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 21Afor a second applied voltage that is greater than the first appliedvoltage;

FIG. 22A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising a negative C-plate passive compensation retarderand homeotropically aligned switchable liquid crystal retarder arrangedbetween the output polariser and additional polariser; and a negativeC-plate passive compensation retarder and homeotropically alignedswitchable liquid crystal retarder arranged between the first-mentionedadditional polariser and further additional polariser in a privacy modeof operation;

FIG. 22B is a schematic diagram illustrating in perspective side view anarrangement of first switchable compensated retarder arranged on theinput of a liquid crystal display and a second switchable compensatedretarder arranged on the output of a liquid crystal display;

FIG. 22C is a schematic diagram illustrating in side perspective view aview angle control optical element comprising a first passivecompensation retarder, a first switchable liquid crystal retarder, afirst control polariser, a second passive compensation retarder, asecond switchable liquid crystal retarder and a second controlpolariser;

FIG. 22D is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for day-time and/or sharing modes of operation;

FIG. 22E is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for day-time and/or sharing modes of operation;

FIG. 22F is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for night-time and/or entertainment modes of operation;

FIG. 22G is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display arranged within thevehicle cabin for night-time and/or entertainment modes of operation;

FIG. 23A is a schematic diagram illustrating in perspective side view anarrangement of a reflective additional polariser and a passive retarderarranged on the input of a liquid crystal display and a switchablecompensated retarder and additional polariser arranged on the output ofa liquid crystal display;

FIG. 23B is a schematic diagram illustrating in side perspective view aview angle control optical element comprising a passive retarder, afirst control polariser, a passive compensation retarder, a switchableliquid crystal retarder and a second control polariser;

FIG. 24A is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarder comprising a negative O-plate tiltedin a plane orthogonal to the display polariser electric vectortransmission direction and a negative C-plate and arranged to providefield-of-view modification of a display device;

FIG. 24B is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in thepassive retarder of FIG. 24A;

FIG. 24C is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarder comprising crossed A-plates and apositive O-plate;

FIG. 24D is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in thepassive retarder of FIG. 24C;

FIG. 24E is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarder comprising two pairs of crossedA-plates;

FIG. 24F is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in thepassive retarder of FIG. 24E;

FIG. 25A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising a negative C-plate passive compensation retarderand homeotropically aligned switchable liquid crystal retarder furthercomprising a patterned electrode layer;

FIG. 25B is a schematic diagram illustrating in perspective front viewillumination of a primary viewer and a snooper by a camouflagedluminance controlled privacy display;

FIG. 25C is a schematic diagram illustrating in perspective side viewillumination of a snooper by a camouflaged luminance controlled privacydisplay;

FIG. 26A is a schematic diagram illustrating in front perspective view adirectional backlight;

FIG. 26B is a schematic diagram illustrating in front perspective view anon-directional backlight;

FIG. 26C is a schematic graph illustrating variation with luminance withlateral viewing angle of displays with different fields of view;

FIG. 27A is a schematic diagram illustrating in side view a switchabledirectional display apparatus comprising an imaging waveguide andswitchable liquid crystal retarder;

FIG. 27B is a schematic diagram illustrating in rear perspective viewoperation of an imaging waveguide in a narrow angle mode of operation;

FIG. 27C is a schematic graph illustrating a field-of-view luminanceplot of the output of FIG. 27B when used in a display apparatus with noswitchable liquid crystal retarder;

FIG. 28A is a schematic diagram illustrating in side view a switchabledirectional display apparatus comprising a switchable collimatingwaveguide and a switchable liquid crystal retarder operating in aprivacy mode of operation;

FIG. 28B is a schematic diagram illustrating in top view output of acollimating waveguide;

FIG. 28C is a schematic graph illustrating an iso-luminancefield-of-view polar plot for the display apparatus of FIG. 28A;

FIG. 29A is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light;

FIG. 29B is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light of a first linearpolarization state at 0 degrees;

FIG. 29C is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light of a first linearpolarization state at 90 degrees;

FIG. 29D is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light of a first linearpolarization state at 45 degrees;

FIG. 30A is a schematic diagram illustrating in perspective viewillumination of a C-plate retarder by off-axis polarised light with apositive elevation;

FIG. 30B is a schematic diagram illustrating in perspective viewillumination of a C-plate retarder by off-axis polarised light with anegative lateral angle;

FIG. 30C is a schematic diagram illustrating in perspective viewillumination of a C-plate retarder by off-axis polarised light with apositive elevation and negative lateral angle;

FIG. 30D is a schematic diagram illustrating in perspective viewillumination of a C-plate retarder by off-axis polarised light with apositive elevation and positive lateral angle;

FIG. 30E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.30A-D;

FIG. 31A is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation;

FIG. 31B is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a negative lateral angle;

FIG. 31C is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and negative lateral angle;

FIG. 31D is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and positive lateral angle; and

FIG. 31E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.31A-D.

DETAILED DESCRIPTION

Terms related to optical retarders for the purposes of the presentdisclosure will now be described.

In a layer comprising a uniaxial birefringent material there is adirection governing the optical anisotropy whereas all directionsperpendicular to it (or at a given angle to it) have equivalentbirefringence.

Optical axis refers to the direction of propagation of a light ray inthe uniaxial birefringent material in which no birefringence isexperienced. For light propagating in a direction orthogonal to theoptical axis, the optical axis is the slow axis when linearly polarizedlight with an electric vector direction parallel to the slow axistravels at the slowest speed. The slow axis direction is the directionwith the highest refractive index at the design wavelength. Similarlythe fast axis direction is the direction with the lowest refractiveindex at the design wavelength.

For positive dielectric anisotropy uniaxial birefringent materials theslow axis direction is the extraordinary axis of the birefringentmaterial. For negative dielectric anisotropy uniaxial birefringentmaterials the fast axis direction is the extraordinary axis of thebirefringent material.

The terms half a wavelength and quarter a wavelength refer to theoperation of a retarder for a design wavelength λ₀ that may typically bebetween 500 nm and 570 nm. In the present illustrative embodimentsexemplary retardance values are provided for a wavelength of 550 nmunless otherwise specified.

The retarder provides a phase shift between two perpendicularpolarization components of the light wave incident thereon and ischaracterized by the amount of relative phase, Γ, that it imparts on thetwo polarization components; which is related to the birefringence Δnand the thickness d of the retarder by

Γ=2·π·Δn·d/λ ₀   eqn. 1

In eqn. 1, Δn is defined as the difference between the extraordinary andthe ordinary index of refraction, i.e.

Δn=n _(e) −n _(o)   eqn. 2

For a half wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is Γ=π.For a quarter wave retarder, the relationship between d, Δn, and λ₀ ischosen so that the phase shift between polarization components is Γ=π/2.

The term half wave retarder herein typically refers to light propagatingnormal to the retarder and normal to the spatial light modulator.

In the present disclosure an ‘A-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisparallel to the (x-y) plane of the layer.

A ‘positive A-plate’ refers to positively birefringent A-plates, i.e.A-plates with a positive Δn.

In the present disclosure a ‘C-plate’ refers to an optical retarderutilizing a layer of birefringent material with its optical axisperpendicular to the plane of the layer. A ‘positive C-plate’ refers topositively birefringent C-plate, i.e. a C-plate with a positive Δn. A‘negative C-plate’ refers to a negatively birefringent C-plate, i.e. aC-plate with a negative Δn.

‘O-plate’ refers to an optical retarder utilizing a layer ofbirefringent material with its optical axis having a component parallelto the plane of the layer and a component perpendicular to the plane ofthe layer. A ‘positive O-plate’ refers to positively birefringentO-plates, i.e. O-plates with a positive Δn.

Achromatic retarders may be provided wherein the material of theretarder is provided with a retardance Δn. d that varies with wavelengthλ as

Δn·d/λ=κ  eqn. 3

where κ is substantially a constant.

Examples of suitable materials include modified polycarbonates fromTeijin Films. Achromatic retarders may be provided in the presentembodiments to advantageously minimise colour changes between polarangular viewing directions which have low luminance reduction and polarangular viewing directions which have increased luminance reductions aswill be described below.

Various other terms used in the present disclosure related to retardersand to liquid crystals will now be described.

A liquid crystal cell has a retardance given by Δn·d where Δn is thebirefringence of the liquid crystal material in the liquid crystal celland d is the thickness of the liquid crystal cell, independent of thealignment of the liquid crystal material in the liquid crystal cell.

Homogeneous alignment refers to the alignment of liquid crystals inswitchable liquid crystal displays where molecules align substantiallyparallel to a substrate. Homogeneous alignment is sometimes referred toas planar alignment. Homogeneous alignment may typically be providedwith a small pre-tilt such as 2 degrees, so that the molecules at thesurfaces of the alignment layers of the liquid crystal cell are slightlyinclined as will be described below. Pretilt is arranged to minimisedegeneracies in switching of cells.

In the present disclosure, homeotropic alignment is the state in whichrod-like liquid crystalline molecules align substantiallyperpendicularly to the substrate. In discotic liquid crystalshomeotropic alignment is defined as the state in which an axis of thecolumn structure, which is formed by disc-like liquid crystallinemolecules, aligns perpendicularly to a surface. In homeotropicalignment, pretilt is the tilt angle of the molecules that are close tothe alignment layer and is typically close to 90 degrees and for examplemay be 88 degrees.

Liquid crystal molecules with positive dielectric anisotropy areswitched from a homogeneous alignment (such as an A-plate retarderorientation) to a homeotropic alignment (such as a C-plate or O-plateretarder orientation) by means of an applied electric field.

Liquid crystal molecules with negative dielectric anisotropy areswitched from a homeotropic alignment (such as a C-plate or O-plateretarder orientation) to a homogeneous alignment (such as an A-plateretarder orientation) by means of an applied electric field.

Rod-like molecules have a positive birefringence so that n_(e)>n_(o) asdescribed in equation 2. Discotic molecules have negative birefringenceso that n_(e)<n_(o).

Positive retarders such as A-plates, positive O-plates and positiveC-plates may typically be provided by stretched films or rod-like liquidcrystal molecules. Negative retarders such as negative C-plates may beprovided by stretched films or discotic like liquid crystal molecules.

Parallel liquid crystal cell alignment refers to the alignment directionof homogeneous alignment layers being parallel or more typicallyantiparallel. In the case of pre-tilted homeotropic alignment, thealignment layers may have components that are substantially parallel orantiparallel. Hybrid aligned liquid crystal cells may have onehomogeneous alignment layer and one homeotropic alignment layer. Twistedliquid crystal cells may be provided by alignment layers that do nothave parallel alignment, for example oriented at 90 degrees to eachother.

Transmissive spatial light modulators may further comprise retardersbetween the input display polariser and the output display polariser forexample as disclosed in U.S. Pat. No. 8,237,876, which is hereinincorporated by reference in its entirety. Such retarders (not shown)are in a different place to the passive retarders of the presentembodiments. Such retarders compensate for contrast degradations foroff-axis viewing locations, which is a different effect to the luminancereduction for off-axis viewing positions of the present embodiments.

Optical isolation retarders provided between the display polariser andan OLED display emission layer are described further in U.S. Pat. No.7,067,985. Optical isolation retarders are in a different place to thepassive retarders of the present embodiments. Isolation retarder reducesfrontal reflections from the OLED display emission layer which is adifferent effect to the luminance reduction for off-axis viewingpositions of the present embodiments.

The structure and operation of various switchable display devices willnow be described. In this description, common elements have commonreference numerals. It is noted that the disclosure relating to anyelement applies to each device in which the same or correspondingelement is provided. Accordingly, for brevity such disclosure is notrepeated.

FIG. 1A is a schematic diagram illustrating in side perspective view anoptical stack of a display device.

Display device 100 comprises a spatial light modulator 48 comprising atleast one display polariser that is the output polariser 218. Backlight20 is arranged to output light and the spatial light modulator 48comprises a transmissive spatial light modulator 48 arranged to receiveoutput light from the backlight 20. The display device 100 is arrangedto output light 400 with angular luminance properties as will bedescribed herein.

In the present disclosure, the spatial light modulator 48 may comprise aliquid crystal display comprising substrates 212, 216, and liquidcrystal layer 214 having red, green and blue pixels 220, 222, 224. Thespatial light modulator 48 has an input display polariser 210 and anoutput display polariser 218 on opposite sides thereof. The outputdisplay polariser 218 is arranged to provide high extinction ratio forlight from the pixels 220, 222, 224 of the spatial light modulator 48.Typical polarisers 210, 218 may be absorbing polarisers such as dichroicpolarisers.

Optionally a reflective polariser 208 may be provided between thedichroic input display polariser 210 and backlight 210 to providerecirculated light and increase display efficiency. Advantageouslyefficiency may be increased.

Backlight 20 may comprise input light sources 15, waveguide 1, rearreflector 3 and optical stack 5 comprising diffusers, light turningfilms and other known optical backlight structures. Asymmetricdiffusers, that may comprise asymmetric surface relief features forexample, may be provided in the optical stack 5 with increased diffusionin the elevation direction in comparison to the lateral direction may beprovided. Advantageously image uniformity may be increased.

In the present embodiments, the backlight 20 may be arranged to providean angular light distribution that has reduced luminance for off-axisviewing positions in comparison to head-on luminance as will bedescribed in FIGS. 26A to 28C below. Backlight 20 may further comprise aswitchable backlight arranged to switch the output angular luminanceprofile in order to provide reduced off-axis luminance in a privacy modeof operation and higher off-axis luminance in a wide angle mode ofoperation. Such switching backlight 20 may cooperate with the switchablecompensated retarder 300 of the present embodiments.

Additional polariser 318 is arranged on the same output side of thespatial light modulator 48 as the display output polariser 218 which maybe an absorbing dichroic polariser.

The display polariser 218 and the additional polariser 318 have electricvector transmission directions 219, 319 that are parallel. As will bedescribed below, such parallel alignment provides high transmission forcentral viewing locations.

Plural retarders which together are referred to herein as a switchablecompensated retarder 300 are arranged between the additional polariser318 and the display polariser 218 and comprise: (i) a switchable liquidcrystal retarder 301 comprising a layer 314 of liquid crystal materialarranged between the display polariser 218 and the additional polariser318; and (ii) a passive compensation retarder 330.

FIG. 1B is a schematic diagram illustrating in front view alignment ofoptical layers in the optical stack of FIG. 1A. The input electricvector transmission direction 211 at the input display polariser 210 ofthe spatial light modulator 48 provides an input polarisation componentthat may be transformed by the liquid crystal layer 214 to provideoutput polarisation component determined by the electric vectortransmission direction 219 of the output display polariser 218. Passivecompensation retarder 330 may comprise retardation layer with a discoticbirefringent material 430, while switchable liquid crystal retarder 301may comprise liquid crystal material.

Switchable compensated retarder 300 thus comprises a switchable liquidcrystal retarder 301 comprising a switchable liquid crystal retarder301, substrates 312, 316 and passive compensation retarder 330 arrangedbetween and additional polariser 318 and display polariser 218.

Substrates 312, 316 may be glass substrates or polymer substrates suchas polyimide substrates. Flexible substrates that may be convenientlyprovided with transparent electrodes may be provided. Advantageouslycurved, bent and foldable displays may be provided.

The display device 100 further comprises a control system 352 arrangedto control the voltage applied by voltage driver 350 across theelectrodes of the switchable liquid crystal retarder 301.

It may be desirable to provide reduced stray light or privacy control ofan emissive display.

FIG. 1C is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising an emissivespatial light modulator 48 and a switchable compensated retarder 300arranged on the output side of the emissive spatial light modulator 48.

Spatial light modulator 48 may alternatively be provided by otherdisplay types that provide output light 400 by emission, such as organicLED displays (OLED), with output display polariser 218, substrates 512,516 and light emission layer 514. Output polariser 218 may providereduction of luminance for light reflected from the OLED pixel plane bymeans of one of more retarders 518 inserted between the output displaypolariser 218 and OLED pixel plane. The one or more retarders 518 may bea quarter waveplate and is different to the compensation retarder 330 ofthe present disclosure.

In the embodiment of FIG. 1C, the spatial light modulator 48 thuscomprises an emissive spatial light modulator and the display polariseris output display polariser 218.

Otherwise, the directional display device of FIG. 1C is the same as thatof FIG. 1A, as described above.

A view angle control optical element 260 for application to a displaydevice will now be described. View angle control optical elements 260may be added to spatial light modulators comprising a display polariser210, 218 to achieve switchable field-of-view characteristics.

FIG. 1D is a schematic diagram illustrating in side perspective view aview angle control optical element 260 for application to a displaydevice comprising a passive compensation retarder 330, a switchableliquid crystal retarder 301 and a control polariser 250.

In use, view angle control optical element 260 may be attached by a useror may be factory fitted to a polarised output spatial light modulator48. View angle control optical element 260 may be provided as a flexiblefilm for curved and bent displays. Alternatively the view angle controloptical element 260 may be provided on a rigid substrate such as a glasssubstrate.

Advantageously, an after-market privacy control element and/or straylight control element may be provided that does not require matching tothe panel pixel resolution to avoid Moire artefacts. View angle controloptical element 260 may be further provided for factory fitting tospatial light modulator 48.

By attaching the view angle control optical element 260 of FIG. 1D to anexisting display device, it is possible to form a display device asshown in any of FIGS. 1A-C.

The embodiments of FIGS. 1A-D provide polar luminance control for light400 that is output from the spatial light modulator 48. That is, theswitchable compensated retarder 300 (comprising the switchable liquidcrystal retarder 301 and the passive compensation retarder 330) does notaffect the luminance of light passing through the input displaypolariser 210, the switchable compensated retarder 300 and theadditional polariser 318 along an axis along a normal to the plane ofthe switchable compensated retarder 300, but the switchable compensatedretarder 300 does reduce the luminance of light passing therethroughalong an axis inclined to a normal to the plane of the switchablecompensated retarder 300, at least in one of the switchable states ofthe compensated switchable retarder 300. The principles leading to thiseffect are described in greater detail below with reference to FIGS.29A-31E and arises from the presence or absence of a phase shiftintroduced by the switchable liquid crystal retarder 301 and the passivecompensation retarder 330 to light along axes that are angleddifferently with respect to the liquid crystal material of theswitchable liquid crystal retarder 301 and the passive compensationretarder 330. A similar effect is achieved in all the devices describedbelow.

Furthermore, the provision of the passive compensation retarder 330 inaddition to the switchable liquid crystal retarder 301 improves theperformance, as will be described in more detail with reference to somespecific display devices, and by comparison to some comparative examplesdescribed with reference to FIGS. 19A-E.

It may be desirable to reduce the number of optical layers between aspatial light modulator 48 and an observer. An arrangement wherein theplural retarders 300 are arranged on the input side of the spatial lightmodulator 48 will now be described.

FIG. 2A is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight 20,a switchable rear retarder 300, a transmissive spatial light modulator48 wherein the additional polariser 318 comprises a reflectivepolariser; and FIG. 2B is a schematic diagram illustrating in front viewalignment of optical layers in the optical stack of FIG. 2A.

The display device 100 comprises a spatial light modulator 48; a displaypolariser 210 arranged on the input side of the spatial light modulator48. Additional polariser 318 is arranged on the same side of the spatiallight modulator 48 as the display polariser 210. Additional polariser318 is a reflective polariser that operates in cooperation with thebacklight 20 to achieve increased efficiency.

Plural retarders 300 are arranged between the reflective additionalpolariser 318 and the display polariser 210. As for FIG. 1A, the pluralretarders 300 comprise: a switchable liquid crystal retarder 301comprising a layer 314 of liquid crystal material arranged between thedisplay polariser 210 and the reflective additional polariser 318; and apassive compensation retarder 330. Thus the reflective additionalpolariser 318 is arranged on the input side of the input displaypolariser 210 between the input display polariser 210 and the backlight20 and the plural retarders 300 are arranged between the reflectiveadditional polariser 318 and the input display polariser 210.

The electric vector transmission direction 319 of the reflectiveadditional polariser 318 is parallel to the electric vector transmissiondirection 211 of input polariser 210 to achieve the switchabledirectional properties as will be described hereinbelow.

In alternative embodiments the additional polariser 318 may compriseboth a reflective polariser and an absorbing dichroic polariser or maycomprise only a dichroic polariser.

The reflective additional polariser 318 may for example be a multilayerfilm such as DBEF™ from 3M Corporation, or may be a wire grid polariser.Advantageously display efficiency may be improved due to light recyclingfrom the polarised reflection from the polariser 372. Further cost andthickness may be reduced in comparison to using both an absorbingdichroic polariser and a reflective polariser as additional polariser318.

In comparison to the arrangement of FIG. 1A, FIG. 2A may provideimproved front of screen image contrast due to the reduced number oflayers between the pixels 220, 222, 224 and an observer.

FIG. 2C is a schematic diagram illustrating in side perspective view anoptical stack of a directional display device comprising a backlight 20,a rear switchable compensated retarder 300, and a transmissive spatiallight modulator 48 wherein the additional polariser 318 comprises adichroic polariser. In comparison to the reflective additional polariser318 of FIG. 2A, the dichroic additional polariser 318 does not recyclehigh angle light into the backlight and thus may reduce the off-axisluminance in comparison to the arrangement of FIG. 2A. Advantageouslyprivacy performance is improved.

The arrangement and operation of the switchable compensated retarders300 and additional polariser 318 of FIGS. 1A-1C and FIGS. 2A-2B will nowbe described.

FIG. 3 is a schematic diagram illustrating in side view an illustrativearrangement of a switchable liquid crystal retarder 301 comprising alayer 314 of liquid crystal material 414 with a negative dielectricanisotropy. Substrates 312, 316 may have transparent electrodes 413, 415arranged thereon and homeotropic surface alignment layers 409, 411arranged on opposite sides of the switchable liquid crystal retarder301. The homeotropic alignment layers 409, 411 may provide homeotropicalignment in the adjacent liquid crystal material 414 with a pretiltangle 407.

The orientation of the liquid crystal material 414 in the x-y plane isdetermined by the pretilt direction of the alignment layers so that eachalignment layer has a pretilt wherein the pretilt of each alignmentlayer has a pretilt direction with a component 417 a, 417 b in the planeof the switchable liquid crystal retarder 301 that is parallel oranti-parallel or orthogonal to the electric vector transmissiondirection 303 of the output display polariser 218.

The pretilt 407 a, 407 b may for example be 88 degrees so that thecomponent 417 is small to achieve reduction of disclinations in therelaxed (zero voltage) state of alignment of the layer 314 of liquidcrystal material 414. Thus the layer 314 is provided by substantially apositive C-plate in the zero voltage arrangement. In practice the liquidcrystal layer further has small O-plate characteristics provided by thehomeotropic alignment layer pretilt at angle 407 a and residualcomponent 417.

The switchable liquid crystal retarder 301 comprises electrodes 413, 415disposed adjacent to the retarder switchable liquid crystal retarder 301and on opposite sides of the switchable liquid crystal retarder 301. Thelayer 314 of liquid crystal material 414 is switchable by means of avoltage being applied across the electrodes 413, 415.

In the undriven state the liquid crystal material 414 is aligned with acomponent 418 perpendicular to the plane of the retarder 301 and acomponent 417 in the plane of the retarder.

The retarder 330 is illustrated as comprising a negative passive O-platecomprising discotic birefringent material 430. The retardance of thepassive compensation retarder 330 may be equal and opposite to theretardance of the switchable liquid crystal retarder 301. The switchableliquid crystal retarder 301 may comprise first and second pretilts 407a, 407 b; and the passive compensation retarder 330 comprises acompensation retarder with first and second pretilts 405 a, 405 b, thefirst pretilt 405 a of the compensation retarder 330 being the same asthe first pretilt 407 a of the liquid crystal retarder 301 and thesecond pretilt 405 b of the compensation retarder 330 being the same asthe second pretilt 307 b of the liquid crystal retarder 301.

Passive O-plates may comprise for example cured reactive mesogen layersthat may be discotic reactive mesogens. Pretilt of the compensationretarder may be achieved by curing reactive mesogen materials afteralignment with a suitable alignment layer. O-plates may also comprisedouble stretched polymer films such as polycarbonate.

In operation, the switchable liquid crystal retarder 301 is switchablebetween two orientation states. The first state may provide displayviewing by multiple viewers. The second state may be provided with anarrow angle mode for privacy operation, or reduced stray light, forexample in night-time operation. As will be described further below,such elements can provide high transmission for a wide range of polarangles in wide angle mode of operation and a restricted luminance polarfield of view in a privacy mode of operation.

The operation of the display of FIG. 1A in wide angle mode representinga first state will now be described.

FIG. 4A is a schematic diagram illustrating in perspective side view anarrangement of the switchable compensated retarder 300 in a wide anglemode of operation. Zero volts is provided across the switchable liquidcrystal retarder 301. In FIG. 4A and other schematic diagrams below,some layers of the optical stack are omitted for clarity. For examplethe switchable liquid crystal retarder 301 is shown omitting thesubstrates 312, 316.

The switchable liquid crystal retarder 301 comprises two surfacealignment layers disposed adjacent to the liquid crystal material 414 onopposite sides thereof and arranged to provide homeotropic alignment atthe adjacent liquid crystal material 414. As described above, the liquidcrystal material 414 may be provided with a pretilt, for example 88degrees from the horizontal to remove degeneracy of liquid crystalmaterial 414 alignment.

The passive compensation retarder 330 comprises a negative C-plateretarder having an optical axis that is a fast axis perpendicular to theplane of the retarder. Thus the material 430 of the C-plate retarder mayhave a negative dielectric anisotropy. C-plates may comprise transparentbirefringent materials such as: polycarbonates or reactive mesogens thatare cast onto a substrate that provides homeotropic alignment forexample; Zeonex™ Cyclo Olefin Polymer (COP); discotic polymers; andNitto Denko™ double stretched polycarbonates.

FIG. 4B is a schematic diagram illustrating a graph of liquid crystaldirector angle 407 against fractional location 440 through theswitchable liquid crystal retarder cell, where the fractional location440 varies between 0 for a location at the surface alignment layer 409and 1 for a location at the surface alignment layer 411.

For a vertically aligned mode with no voltage applied as illustrated inFIG. 4A, the liquid crystal directors are at a tilt 407 of 88 degreesthrough the thickness of the cell as indicated by tilt profile 442. Thetilt profile for the layer 314 may be the same as the profile 442. Thecompensation retarder 330 may provide correction for the pretiltdirection of the switchable liquid crystal retarder 301. Thecompensation retarder 330 may alternatively have a uniform tilt angle of90 degrees, such difference from the pretilt of the liquid crystal layerproviding only small difference in off-axis viewing properties.

Thus the off-axis retardance of the compensation retarder 330 issubstantially equal and opposite to the off-axis retardance of theswitchable liquid crystal retarder 301 when no voltage is applied.

FIG. 4C is a schematic diagram illustrating in side view propagation ofoutput light from the spatial light modulator 48 through the opticalstack of FIG. 1A in a wide angle mode of operation; and FIG. 4D is aschematic graph illustrating the variation of output transmission withpolar direction for the transmitted light rays in FIG. 4C in a wideangle mode of operation.

An ideal compensated switchable retarder 300 comprises compensationretarder 330 in combination with a variable switchable liquid crystalretarder 301 wherein the dielectric constants, anisotropy and dispersionof anisotropy of the compensation retarder 330 have the equal andopposite dielectric constants, anisotropy and dispersion of anisotropyto that of the layer 314. The retardance of the passive compensationretarder 330 is equal and opposite to the retardance of the switchableliquid crystal retarder 301.

Such an ideal compensated switchable retarder achieves compensation fortransmitted light in a first wide angle state of the layer 314 of liquidcrystal material 414 for all polar angles; and narrow field of view in alateral direction in a second privacy state of the switchable liquidcrystal retarder 301.

Further the optical axis of compensation retarder 330 has the samedirection as that of the optical axis of the liquid crystal retarder 301in its wide angle state. Such a compensation retarder 330 cancels outthe retardation of the liquid crystal retarder for all viewing angles,and provides an ideal wide angle viewing state with no loss of luminancefor all viewing directions.

The wide angle transmission polar profile for non-ideal materialselections will now be described.

The illustrative embodiments of the present disclosure illustratecompensation retarders 330 that may not exactly compensate theretardation of the switchable liquid crystal retarder 301 because ofsmall differences in material properties that are typical for theretarders 330, 301. However, advantageously such deviations are smalland high performance wide and narrow angle states can be achieved withsuch deviations that may be close to ideal performance.

Thus when the switchable liquid crystal retarder 301 is in a first stateof said two states, the switchable compensated retarder 300 provides nooverall transformation of polarisation component 360, 361 to outputlight rays 400 passing therethrough perpendicular to the plane of theswitchable retarder or at an acute angle to the perpendicular to theplane of the switchable retarder, such as for light rays 402.

Polarisation component 362 is substantially the same as polarisationcomponent 360 and polarisation component 364 is substantially the sameas polarisation component 361. Thus the angular transmission profile ofFIG. 4D is substantially uniformly transmitting across a wide polarregion.

In other words, when the layer of liquid crystal material 414 is in thefirst orientation state of said two orientation states, the pluralretarders 330, 301 provide no overall retardance to light passingtherethrough perpendicular to the plane of the retarders or at an acuteangle to the perpendicular to the plane of the retarders 330, 301.

Advantageously the variation of display luminance with viewing angle inthe first state is substantially unmodified. Multiple users mayconveniently view the display from a wide range of viewing angles.

The operation of the compensated retarder 300 and additional polariser318 in a narrow angle mode for example for use in a privacy mode ofoperation will now be described.

FIG. 5A is a schematic diagram illustrating in perspective side view anarrangement of the switchable compensated retarder 300 in a privacy modeof operation comprising a negative C-plate passive compensation retarder330 and homeotropically aligned switchable liquid crystal retarder 301in a privacy mode of operation.

The liquid crystal retarder 301 further comprises transparent electrodes413, 415 such as ITO electrodes arranged across the switchable liquidcrystal retarder 301. Electrodes 413, 415 control the switchable liquidcrystal retarder 301 by adjusting the voltage being applied to theelectrodes 413, 415.

Control system 352 is arranged to control the voltage applied by voltagedriver 350 across the electrodes 413, 415 of the switchable liquidcrystal retarder 301.

Returning to FIG. 4B, when a voltage is applied the splayed tilt profile444 of is provided for switchable liquid crystal retarder 301 such thatthe retardance of the layer 314 of liquid crystal material 414 ismodified.

The direction of optimum privacy performance may be adjusted in responseto observer position by control of the drive voltage. In another use orto provide controlled luminance to off-axis observers for example in anautomotive environment when a passenger or driver may wish somevisibility of the displayed image, without full obscuration, by means ofintermediate voltage levels.

FIG. 5B is a schematic diagram illustrating in side view propagation ofoutput light from the spatial light modulator 48 through the opticalstack of FIG. 1A in a privacy mode of operation wherein the switchableliquid crystal retarder 301 is oriented by means of an applied voltage.

In the present embodiments, the compensated switchable liquid crystalretarder 330 may be configured, in combination with the displaypolariser 210, 218, 316 and the additional polariser 318, to have theeffect that the luminance of light output from the display device at anacute angle to the optical axis (off-axis) is reduced, i.e. compared tothe retarder not being present. The compensated switchable liquidcrystal retarder 330 may also be configured, in combination with thedisplay polariser 210, 218, 316 and the additional polariser 318, tohave the effect that the luminance of light output from the displaydevice along the optical axis (on-axis) is not reduced, i.e. compared tothe retarder not being present.

Polarisation component 360 from the output display polariser 218 istransmitted by output display polariser 218 and incident on switchablecompensated retarder 300. On-axis light has a polarisation component 362that is unmodified from component 360 while off-axis light has apolarisation component 364 that is transformed by retarders ofswitchable compensated retarder 300. At a minimum, the polarisationcomponent 361 is transformed to a linear polarisation component 364 andabsorbed by additional polariser 318. More generally, the polarisationcomponent 361 is transformed to an elliptical polarisation component,that is partially absorbed by additional polariser 318.

Thus when the retarder switchable liquid crystal retarder 301 is in thesecond orientation state of said two orientation states, the pluralretarders 301, 330 provide no overall retardance to light passingtherethrough along an axis perpendicular to the plane of the retarders,but provides a non-zero overall retardance to light passing therethroughfor some polar angles 363 that are at an acute angle to theperpendicular to the plane of the retarders 301, 330.

In other words when the switchable liquid crystal retarder 301 is in asecond state of said two states, the switchable compensated retarder 330provides no overall transformation of polarisation component 360 tooutput light rays 400 passing therethrough along an axis perpendicularto the plane of the switchable retarder 301, but provides an overalltransformation of polarisation component 361 to light rays 402 passingtherethrough for some polar angles which are at an acute angle to theperpendicular to the plane of the retarders 301, 330.

An illustrative material system will be described for narrow angleoperation.

FIG. 5C is a schematic graph illustrating the variation of outputtransmission with polar direction for the transmitted light rays in FIG.5B, with the parameters described in TABLE 1.

TABLE 1 Passive compensation retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ Voltage/ FIG. Mode Type nm layers deg nm Δϵ V4A & 4D Wide Negative C −700 Homeotropic 88 810 −4.3 0 5A & 5C PrivacyHomeotropic 88 2.2

In the present embodiments, desirable ranges for retardations andvoltages have been established by means of simulation of retarder stacksand experiment with display optical stacks.

The switchable liquid crystal retarder 300 comprises a first surfacealignment layer 409 disposed on a first side of the layer of liquidcrystal material 414, and a second surface alignment layer 411 disposedon the second side of the layer of liquid crystal material 414 oppositethe first side; wherein the first surface alignment layer 409 is ahomeotropic alignment layer and the second surface alignment layer 411is a homeotropic alignment layer, wherein the layer of liquid crystalmaterial has an retardance for light of a wavelength of 550 nm between500 nm and 1000 nm, preferably between 600 nm and 900 nm and mostpreferably between 700 nm and 850 nm.

When the passive compensation retarder 330 comprises a retarder havingan optical axis perpendicular to the plane of the retarder, the passiveretarder has a retardance for light of a wavelength of 550 nm between−300 nm and −900 nm, preferably between −450 nm and −800 nm and mostpreferably between −500 nm and −725 nm.

The polar distribution of light transmission illustrated in FIG. 5Cmodifies the polar distribution of luminance output from the underlyingspatial light modulator 48 and where applicable the backlight 20.

Advantageously, a privacy display is provided that has low luminance toan off-axis snooper while maintaining high luminance for an on-axisobserver. A large polar region is provided over which the luminance ofthe display to an off-axis snooper is reduced. Further the on-axisluminance is substantially unaffected for the primary display user inprivacy mode of operation.

The voltage applied across the electrodes is zero for the firstorientation state and non-zero for the second orientation state.Advantageously the wide mode of operation may have no additional powerconsumption, and the failure mode for driving of the switchable liquidcrystal retarder 301 is for wide angle mode.

The operation of the privacy mode of the display of FIG. 1A will now bedescribed further.

FIG. 6A is a schematic diagram illustrating in front perspective viewobservation of transmitted output light for a display operating inprivacy mode. Display device 100 may be provided with white regions 603and black regions 601. A snooper may observe an image on the display ifluminance difference between the observed regions 601, 603 can beperceived. In operation, primary user 45 observes a full luminanceimages by rays 400 to viewing locations 26 that may be optical windowsof a directional display. Snooper 47 observes reduced luminance rays 402in viewing locations 27 that may be optical windows of a directionaldisplay. Regions 26, 27 further represent on-axis and off-axis regionsof FIG. 5C.

FIG. 6B is a schematic diagram illustrating in front perspective viewsthe appearance of the display of FIG. 1A operating in privacy mode 1with luminance variations as illustrated in FIG. 5C. Thus upper viewingquadrants 530, 532, lower viewing quadrants 534, 536 and lateral viewingpositions 526, 528 provide reduced luminance, whereas up/down centralviewing regions 522, 520 and head-on viewing provides much higherluminance.

It may be desirable to provide controllable display illumination in anautomotive vehicle.

FIG. 6C is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 of an automotive vehicle 600 for both entertainmentand sharing modes of operation. Light cone 610 (for example representingthe cone of light within which the luminance is greater than 50% of thepeak luminance) may be provided by the luminance distribution of thedisplay 100 in the elevation direction and is not switchable.

FIG. 6D is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in an entertainment mode of operation and operates ina similar manner to a privacy display. Light cone 612 is provided with anarrow angular range such that passenger 606 may see the display 100whereas driver 604 may not see an image on the display 100.Advantageously entertainment images may be displayed to the passenger606 without distraction to the driver 604.

FIG. 6E is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in a sharing mode of operation. Light cone 614 isprovided with a wide angular range such that all occupants may perceivean image on the display 100, for example when the display is not inmotion or when non-distracting images are provided.

FIG. 6F is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 for both night-time and day-time modes of operation.In comparison to the arrangements of FIGS. 6C-E, the optical output isrotated so that the display elevation direction is along an axis betweenthe driver 604 and passenger 606 locations. Light cone 620 illuminatesboth driver 604 and passenger 606.

FIG. 6G is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in a night-time mode of operation. Thus the displaymay provide a narrow angular output light cone 622. Stray light thatilluminates internal surfaces and occupants of the vehicle cabin 602 andcause distraction to driver 604 may advantageously be substantiallyreduced. Both driver 604 and passenger 606 may advantageously be able toobserve the displayed images.

FIG. 6H is a schematic diagram illustrating in side view an automotivevehicle with a switchable directional display 100 arranged within thevehicle cabin 602 in a day-time mode of operation. Thus the display mayprovide a narrow angular output light cone 624. Advantageously thedisplay may be conveniently observed by all cabin 602 occupants.

The displays 100 of FIGS. 6C-H may be arranged at other vehicle cabinlocations such as driver instrument displays, center console displaysand seat-back displays.

FIGS. 7A-D are schematic diagrams illustrating the variation of outputtransmission with polar direction for four different drive voltages from2.05V to 2.35V in 0.1V increments. Thus the applied voltage may providecontrol of the luminance field-of-view minima locations in the privacymode of operation. Further the luminance minima may be controlledbetween an elevation that is zero or less to elevations that are in theupper quadrants of the polar profile.

FIG. 8 is a flow chart illustrating control of a privacy displayimplemented by a control system. The control may be applied to each ofthe devices described herein.

In a first step 870 a user may enable a privacy mode of operation.

Where a first and further compensated switchable liquid crystalretarders 300B are provided (as for example in the device of FIG. 22Adescribed below), the control system is arranged in the secondorientation state to control the voltage applied across the electrodes413, 415 of the first-mentioned switchable liquid crystal retarder 314Aand to control the voltage applied across the electrodes of the furtherswitchable liquid crystal retarder 314B; wherein the overall retardanceto light passing through the first-mentioned switchable liquid crystalretarder 314A and first-mentioned passive compensation retarder 330A atsome polar angles at an acute angle to the perpendicular to the plane ofthe retarders 31A, 330A is different to the overall retardance to lightpassing through the further switchable liquid crystal retarder 314B andfurther passive compensation retarder 330B at the same polar angles.

Such a privacy mode setting may be provided by manual setting (forexample a keyboard operation) or by automatic sensing using sensor tolocate the presence of a snooper as described for example in U.S. PatentPubl. No. 2017-0236494, which is incorporated herein by reference in itsentirety. Optionally the display orientation with respect to the snoopermay be further detected by means of detector 873.

In a second step 872 the snooper location may be detected for example bymeans of a camera or by a keyboard setting or other method. In anillustrative example, an OFFICE setting may be provided wherein it maybe desirable to optimise privacy performance for snoopers that aremoving around a shared office environment and thus optimise performancefor look-down viewing quadrants. By way of comparison in a FLIGHTsetting, it may be desirable to provide privacy level optimisation forsitting snoopers, with improved privacy level for lower elevations thandesirable for OFFICE setting.

In a third step 876 the voltage applied to the switchable liquid crystalretarder 301 may be adjusted and in a fourth step 878 the LED profilemay be adjusted with the control system.

Thus the control system may further comprise a means 872 to determinethe location of a snooper 47 with respect to the display device 100 andthe control system is arranged to adjust the voltage applied by drive350 across the electrodes 413, 415 of the switchable liquid crystalretarder 314 in response to the measured location of the snooper 47.

Advantageously the privacy operation of the display may be controlled tooptimise for snooper viewing geometry.

Returning to the discussion of the present embodiments, furtherarrangements of compensated switchable retarders 300 will now bedescribed.

FIG. 9A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising crossed A-plate passive compensation retarders 308A, 308B andhomeotropically aligned switchable liquid crystal retarder 301; and FIG.9B is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising crossed A-plate passive compensation retarders andhomeotropically aligned switchable liquid crystal retarder.

In comparison to the arrangement of FIG. 4A and FIG. 5A, thecompensation retarder 330 may alternatively comprise a pair of retarders308A, 308B which have optical axes in the plane of the retarders thatare crossed. The compensation retarder 330 thus comprises a pair ofretarders 308A, 308B that each comprise a single A-plate.

The pair of retarders 308A, 308B each comprise plural A-plates havingrespective optical axes 309A, 309B aligned at different angles withrespect to each other. The pair of retarders have optical axes 309A,309B that each extend at 45° with respect to an electric vectortransmission direction that is parallel to the electric vectortransmission direction 211 of the input display polariser 210 in thecase that the additional polariser 318 is arranged on the input side ofthe input display polariser or is parallel to the electric vectortransmission direction 219 of the output display polariser 218 in thecase that the additional polariser 318 is arranged on the output side ofthe input display polariser 218.

FIG. 9C is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 9Ain a wide angle mode of operation; and FIG. 9D is a schematic graphillustrating the variation of output transmission with polar directionfor transmitted light rays in FIG. 9B in a privacy mode of operationprovided by the illustrative embodiment of TABLE 2.

TABLE 2 Passive compensation retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ Voltage/ FIG. Mode Type nm layers deg nm Δϵ V9A & 9C Wide Crossed A +650 @ 45° Homeotropic 88 810 −4.3 0 9B & 9DPrivacy +650 @ 135° Homeotropic 88 2.3

When the passive compensation retarder 330 comprises a pair of retarderswhich have optical axes in the plane of the retarders that are crossed,each retarder of the pair of retarders has a retardance for light of awavelength of 550 nm between 300 nm and 800 nm, preferably between 500nm and 700 nm and most preferably between 550 nm and 675 nm.

Advantageously A-plates may be more conveniently manufactured at lowercost than for the C-plate retarder of FIG. 4A and FIG. 5A. Further azero voltage state may be provided for the wide angle mode of operation,minimising power consumption during wide angle operation.

In the present embodiments, ‘crossed’ refers to an angle ofsubstantially 90° between the optical axes of the two retarders in theplane of the retarders. To reduce cost of retarder materials, it isdesirable to provide materials with some variation of retarderorientation due to stretching errors during film manufacture forexample. Variations in retarder orientation away from preferabledirections can reduce the head-on luminance and increase the minimumtransmission. Preferably the angle 310A is at least 35° and at most 55°,more preferably at least 40° and at most 50° and most preferably atleast 42.5° and at most 47.5°. Preferably the angle 310B is at least125° and at most 145°, more preferably at least 130° and at most 135°and most preferably at least 132.5° and at most 137.5°.

During mechanical distortion, such as when touching the display, thehomeotropically aligned liquid crystal retarders 301 of FIGS. 9A-9B mayhave undesirably long recovery times creating visible misalignmentartefacts. It would be desirable to provide fast recovery times aftermechanical distortion.

FIGS. 10A-10B are schematic diagrams illustrating in perspective sideview an arrangement of a switchable retarder in a wide angle and privacymode of operation respectively comprising a homogeneously alignedswitchable liquid crystal retarder comprising liquid crystal material414 with a positive dielectric anisotropy and a passive negative C-plateretarder 330 for first and second drive voltages respectively.

The switchable liquid crystal retarder further comprises surfacealignment layers 431, 433 disposed adjacent to the layer of liquidcrystal material 414 and each arranged to provide homogeneous alignmentin the adjacent liquid crystal material. In other words, the switchableliquid crystal retarder comprises two surface alignment layers 431, 433disposed adjacent to the layer of liquid crystal material 414 and onopposite sides thereof and each arranged to provide homogeneousalignment in the adjacent liquid crystal material 414.

FIG. 10C is a schematic diagram illustrating a graph of liquid crystaldirector angle 407 against fractional location 440 through theswitchable liquid crystal retarder 301 of FIG. 10A for various differentapplied voltages. FIG. 10C differs from FIG. 4B wherein the pretiltangle is small and increases with applied voltage. Profile 441illustrates liquid crystal material 414 tilt angle for 0V appliedvoltage, tilt profile 443 illustrates director orientations for 2.5V andtilt profile 445 illustrates director orientations for 5V. Thus theliquid crystal layers are typically splayed in desirable switchedstates, and compensated by the compensation retarders 330. Increasingthe voltage above 2.5V to 10V progressively reduces the thickness of theretarder 301 in which splay is present, and advantageously increases thepolar field of view over which the transmission is maximised.

Resolved component 419 a, 419 b of liquid crystal tilt compared to thedirection perpendicular to the plane of the retarder is substantiallyhigher than components 417 a, 417 b of FIG. 5A.

The increased magnitude of resolved component 419 a, 419 b may provideincreased restoring force after mechanical distortion in comparison tothe arrangement of FIG. 9A for example. Sensitivity to mechanicaldistortions such as during touching the display may advantageously bereduced.

The voltage of operation may be reduced below 10V for acceptable wideangle field of view, reducing power consumption; and reducing cost andcomplexity of electrical driving.

FIGS. 11A-11C are schematic graphs illustrating the variation of outputtransmission with polar direction for transmitted light rays ofswitchable compensated retarder comprising a homogeneously alignedliquid crystal retarder 301 and a passive negative C-plate compensationretarder 330, similar to the display device of FIGS. 10A and 10B, in aprivacy mode and two different wide angle modes for different drivevoltages comprising the embodiments illustrated in TABLE 3.

TABLE 3 Passive compensation retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ Voltage/ FIG. Mode Type nm layers deg nm Δϵ V11A Privacy Negative C −500 Homogeneous 2 750 +13.2 2.3 11B WideHomogeneous 2 5.0 11C Wide 10.0

Desirable ranges for optical retardance for active LC retarder 301comprising homogeneous alignment layers 431, 433 on both substrates anda passive negative C-plate compensation retarder 330 are furtherdescribed in TABLE 4.

TABLE 4 Active LC Minimum Typical Maximum layer negative C- negative C-negative C- retardance/ plate plate plate nm retardance/nm retardance/nmretardance/nm 600 −300 −400 −500 750 −350 −450 −600 900 −400 −500 −700

The switchable liquid crystal retarder 300 thus comprises a firstsurface alignment layer 431 disposed on a first side of the layer ofliquid crystal material 414, and a second surface alignment layer 433disposed on the second side of the layer of liquid crystal material 414opposite the first side; wherein the first surface alignment layer 409is a homogeneous alignment layer and the second surface alignment layeris a homogeneous alignment layer; wherein the layer of liquid crystalmaterial has a retardance for light of a wavelength of 550 nm in a rangefrom 500 nm to 1000 nm, preferably in a range from 600 nm to 850 nm andmost preferably in a range from 700 nm to 800 nm. Thus when the firstand second alignment layers are each homogeneous alignment layers andwhen the passive compensation retarder 330 comprises a retarder havingan optical axis perpendicular to the plane of the retarder, the passiveretarder has a retardance for light of a wavelength of 550 nm in a rangefrom −300 nm to −700 nm, preferably in a range from −350 nm to −600 nmand most preferably −400 nm to −500 nm.

Advantageously off-axis privacy can be provided by means of luminancereduction and privacy level increase over wide polar regions. Furtherresistance to visual artefacts arising from flow of liquid crystalmaterial in the layer 314 may be improved in comparison to homeotropicalignment.

Various other configurations of the optical structure and driving ofFIG. 10A will now be described.

Operation at 5V provides lower power consumption and lower costelectronics while achieving acceptable luminance roll-off in wide anglemode. Field of view in wide angle mode can further be extended byoperation at 10V.

FIG. 12A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation, the arrangement comprising crossed A-plate passivecompensation retarders 308A, 308B and homogeneously aligned switchableliquid crystal retarder 301; and FIGS. 12B-D are schematic graphsillustrating the variation of output transmission with polar directionfor transmitted light rays of switchable compensated retarder 301comprising a homogeneously aligned liquid crystal material 414 andpassive crossed A-plate retarders 308A, 308B, in a privacy mode and awide angle mode for different drive voltages comprising the respectiveembodiments illustrated in TABLE 5.

TABLE 5 Passive compensation retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ Voltage/ FIG. Mode Type nm layers deg nm Δϵ V12B Privacy Crossed A +500 @ 45° Homogeneous 2 750 +13.2 2.3 12C Wide+500 @ 135° Homogeneous 2 5 12D Wide 10

Desirable ranges for optical retardance for active LC retarder 301comprising homogeneous alignment layers 409, 411 on both substrates andcrossed positive A-plate retarders 308A, 308B are further described inTABLE 6.

TABLE 6 Active LC Minimum Typical Maximum layer positive positivepositive retardance/ A-plate A-plate A-plate nm retardance/nmretardance/nm retardance/nm 600 +300 +400 +600 750 +350 +500 +700 900+400 +600 +800

Thus when: the first and second alignment layers are each homogeneousalignment layers; the layer of liquid crystal material has a retardancefor light of a wavelength of 550 nm in a range from 500 nm to 1000 nm,preferably in a range from 600 nm to 850 nm and most preferably in arange from 700 nm to 800 nm; and the passive compensation retarder 330comprises a pair of retarders which have optical axes in the plane ofthe retarders that are crossed, then each retarder of the pair ofretarders has a retardance for light of a wavelength of 550 nm between300 nm and 800 nm, preferably between 350 nm and 650 nm and mostpreferably between 450 nm and 550 nm.

Further crossed A-plates may be conveniently provided from low costmaterials.

By way of illustration various other example embodiments of the opticalstructure and driving of FIG. 12A will now be described. FIG. 12C andFIG. 12D further illustrate that by adjustment of addressing voltage andretardances, advantageously different wide angle fields of view may beachieved.

Arrangements of optical stack structures will now be further described.

FIG. 13A and FIG. 13B are schematic diagrams illustrating in side viewspart of a display comprising a switchable compensated retarder andoptical bonding layers 380. Optical bonding layers 380 may be providedto laminate films and substrates, achieving increased efficiency andreduced luminance at high viewing angles in privacy mode. Further an airgap 384 may be provided between the spatial light modulator 48 and theswitchable compensated retarder 300. To reduce wetting of the twosurfaces at the air gap 384, an anti-wetting surface 382 may be providedto at least one of the switchable compensated retarder 300 or spatiallight modulator 48.

The passive compensation retarder 330 may be provided between theswitchable liquid crystal layer 301 and spatial light modulator 48 asillustrated in FIG. 13A, or may be provided between the additionalpolariser 318 and switchable liquid crystal retarder 301 as illustratedin FIG. 13B. Substantially the same optical performance is provided inboth systems.

FIG. 13A illustrates that optical layers are bonded to outer sides ofthe substrates 312, 316. Advantageously, bending of the substrates 312,316 from the attached layers due to stored stresses during laminationmay be reduced and display flatness maintained.

Similarly, switchable compensated retarder 300 may be arranged whereinthe output polariser 218 is the display polariser. Scatter that may beprovided by spatial light modulator 48, such as from phase structures atthe pixels 220, 222, 224 do not degrade the output luminance profile incomparison to arrangements wherein the switchable compensated retarder301 is arranged behind the spatial light modulator 48.

It may be desirable to provide the additional polariser with a differentelectric vector transmission direction to the electric vectortransmission direction of the display polariser.

FIG. 14A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising the crossed A-plate passive compensation retarders308A, 308B and homogeneously aligned switchable liquid crystal retarder301, as described above but further comprising a passive rotationretarder 460.

The display polariser 218 may be provided with an electric vectortransmission direction 219, that may be for example at an angle 317 of45 degrees in the case of a twisted nematic LCD display. The additionalpolariser 318 may be arranged to provide vertically polarised light to auser who may be wearing polarising sunglasses that typically transmitvertically polarised light.

The passive rotation retarder 460 is different to the compensationretarder 330 of the present embodiments and its operation will now bedescribed.

Passive rotation retarder 460 may comprise a birefringent material 462and be a half waveplate, with retardance at a wavelength of 550 nm of275 nm for example.

Passive rotation retarder 460 has a fast axis orientation 464 that isinclined at an angle 466 that may be 22.5 degrees to the electric vectortransmission direction 319 of the additional polariser 318. The passiverotation retarder 460 thus rotates the polarisation from the outputpolariser 218 such that the polarisation direction of the light that isincident onto the compensation retarder 308B is parallel to thedirection 319.

The passive rotation retarder 460 modifies the on-axis polarisationstate, by providing an angular rotation of the polarisation componentfrom the display polariser 218. In comparison the compensation retarders308A, 308B together do not modify the on-axis polarisation state.

Further, the passive rotation retarder 460 provides a rotation ofpolarisation that may be substantially independent of viewing angle. Incomparison the compensation retarders 308A, 308B provide substantialmodifications of output luminance with viewing angle.

Advantageously a display may be provided with an output polarisationdirection 319 that is different from the display polariser polarisationdirection 219, for example to provide viewing with polarisingsunglasses.

In an alternative embodiment the separate retarder 460 may be omittedand the retardance of the retarder 308B of FIG. 11A increased to providean additional half wave rotation in comparison to the retardance ofretarder 308A. To continue the illustrative embodiment, the retardanceof retarder 308B at a wavelength of 550 nm may be 275 nm greater thanthe retardance of retarder 308A. Advantageously the number of layers,complexity and cost may be reduced.

It would be desirable to provide reduced thickness and reduced totalnumber of optical components.

FIG. 15A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder in a privacy mode ofoperation comprising a homogeneously aligned switchable liquid crystalretarder 301 arranged between first and second C-plate passivecompensation retarders 330A, 330B, further illustrated in TABLE 7.

TABLE 7 Passive compensation retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ Voltage/ FIG. Mode Type nm layers deg nm Δϵ V15B Wide Negative C, 330A −275 Homogeneous 2 750 132 5.0 15A & 15CPrivacy Negative C, 330B −275 Homogeneous 2 2.6 17A & 17B Wide A-plate,330A 575 Homogeneous 2 750 132 5.0 17C Privacy A-plate, 330B 575Homogeneous 2 2.6

FIG. 15B and FIG. 15C are schematic graphs illustrating the variation ofoutput transmission with polar direction for transmitted light rays inthe optical stack of FIG. 15A in a wide angle mode and a privacy mode ofoperation respectively.

The passive compensation retarder 330 comprises first and secondC-plates 330A, 330B; and the switchable liquid crystal layer 301 isprovided between the first and second C-plates 330A, 330B.

The passive compensation retarder 330A, 330B comprises two passiveretarders having an optical axis perpendicular to the plane of thepassive retarders, and the switchable liquid crystal retarder 301 isprovided between the two passive retarders. The first and secondsubstrates 312, 316 of FIG. 1A thus each comprise one of the two passiveretarders 330A, 330B.

In combination the two passive retarders 330A, 330B have a totalretardance for light of a wavelength of 550 nm in a range −300 nm to−800 nm, preferably in a range from −350 nm to −700 nm and mostpreferably in a range from −400 nm to −600 nm.

FIG. 16A is a schematic diagram illustrating in perspective side view adisplay comprising a switchable liquid crystal retarder 301 arrangedbetween first and second substrates each comprising C-plate passivecompensation retarders 330A, 330B; and FIG. 16B is a schematic diagramillustrating in side view part of a display comprising a switchableliquid crystal retarder 301 arranged between first and second substrateseach comprising C-plate passive compensation retarders 330A, 330B.

The first C-plate 330A has a transparent electrode layer 415 and liquidcrystal alignment layer 411 formed on one side and the second C-plate330B has a transparent electrode layer 413 and liquid crystal alignmentlayer 409 formed on one side.

The layer 314 of liquid crystal material is provided between first andsecond substrates 312, 316, and the first and second substrates 312, 316each comprises one of the first and second C-plates 330A, 330B. TheC-plates may be provided in double stretched COP films that are ITOcoated to provide electrodes 413, 415 and have liquid crystal alignmentlayers 409, 411 formed thereon.

Advantageously, the number of layers may be reduced in comparison to thearrangement of FIG. 1, reducing thickness, cost and complexity. Furtherthe C-plates 330A, 330B may be flexible substrates, and may provide aflexible privacy display.

It would be desirable to provide a layer 314 of liquid crystal materialbetween first and second A-plate substrates.

FIG. 17A is a schematic diagram illustrating in perspective side view anarrangement of a switchable compensated retarder 300 in a wide anglemode of operation, comprising a homogeneously aligned switchable liquidcrystal retarder 301 arranged between first and second crossed A-platepassive compensation retarders 330A, 330B, as described above; and FIG.17B and FIG. 17C are schematic graphs illustrating the variation ofoutput transmission with polar direction for transmitted light rays forthe structure of FIG. 17A when driven in wide angle and privacy modes ofoperation respectively comprising the further illustrative embodimentsillustrated in TABLE 7.

In comparison to the arrangement of FIG. 15A, advantageously A-platesmay be manufactured at reduced cost compared to C-plates.

Hybrid aligned structures comprising both homogeneous and homeotropicalignment layers will now be described.

FIG. 18A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising a homogeneously and homeotropically aligned switchable liquidcrystal retarder 301 comprising liquid crystal material 423 and apassive negative C-plate retarder 330.

FIGS. 18B-18C are schematic graphs illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG. 18Ain a wide angle and privacy mode of operation respectively, and providedby the arrangement of TABLE 8.

TABLE 8 Passive compensation retarder(s) Active LC retarder Δn · d/Alignment Pretilt/ Δn · d/ Voltage/ FIG. Mode Type nm layers deg nm Δϵ V18C Wide Negative C −1100 Homogeneous 2 1300 +4.3 15.0 18A PrivacyHomeotropic 88 2.8 Not shown Wide Crossed A +1100 @ 45° Homeotropic 21300 +4.3 15.0 Not shown Privacy +1100 @ 135° Homogeneous 88 2.8

The hybrid aligned switchable liquid crystal retarder 301 has variabletilt such that for a given material and cell thickness choice, reducedeffective birefringence is provided. Thus the retarder design must beadjusted to compensate in comparison to the arrangements wherein thealignment layers are the same. The switchable liquid crystal retarder330 comprises a first surface alignment layer 441 disposed on a firstside of the layer of liquid crystal material 423, and a second surfacealignment layer 443 disposed on the second side of the layer of liquidcrystal material 423 opposite the first side. The first surfacealignment layer 441 is a homeotropic alignment layer arranged to providehomeotropic alignment in the adjacent liquid crystal material 423 andthe second surface alignment layer 443 is a homogeneous alignment layerarranged to provide homogeneous alignment in the adjacent liquid crystalmaterial 423.

Further, the optimum designs of retarders are related to the relativelocation of the passive compensation retarder 330 with respect to thehomeotropic and homogeneous alignment layers.

When the surface alignment layer 443 arranged to provide homogeneousalignment is between the layer of liquid crystal material 423 and thecompensation retarder 330, the layer of liquid crystal material 423 hasa retardance for light of a wavelength of 550 nm in a range from 500 nmto 1800 nm, preferably in a range from 700 nm to 1500 nm and mostpreferably in a range from 900 nm to 1350 nm. When the surface alignmentlayer 443 arranged to provide homogeneous alignment is between the layerof liquid crystal material 423 and the compensation retarder 330, thepassive compensation retarder may comprise a retarder 330 having itsoptical axis perpendicular to the plane of the retarder as shown in FIG.18A, the passive retarder 330 having a retardance for light of awavelength of 550 nm in a range from −300 nm to −1600 nm, preferably ina range from −500 nm to —1300 nm and most preferably in a range from−700 nm to −1150 nm; or alternatively the passive compensation retardermay comprise a pair of retarders (not shown) which have optical axes inthe plane of the retarders that are crossed, each retarder of the pairof retarders having a retardance for light of a wavelength of 550 nm ina range from 400 nm to 1600 nm, preferably in a range from 600 nm to1400 nm and most preferably in a range from 800 nm to 1300 nm.

When the surface alignment layer 441 arranged to provide homeotropicalignment is between the layer of liquid crystal material 423 and thecompensation retarder 330, the layer of liquid crystal material 423 hasa retardance for light of a wavelength of 550 nm in a range from 700 nmto 2000 nm, preferably in a range from 1000 nm to 1700 nm and mostpreferably in a range from 1200 nm to 1500 nm. When the surfacealignment layer 441 arranged to provide homeotropic alignment is betweenthe layer of liquid crystal material 423 and the compensation retarder330, the passive compensation retarder may comprise a retarder 330having its optical axis perpendicular to the plane of the retarder asshown in FIG. 18A, the passive retarder having a retardance for light ofa wavelength of 550 nm in a range from −400 nm to −1800 nm, preferablyin a range from −700 nm to −1500 nm and most preferably in a range from−900 nm to −1300 nm; or alternatively the passive compensation retardermay comprise a pair of retarders (not shown) which have optical axes inthe plane of the retarders that are crossed, each retarder of the pairof retarders having a retardance for light of a wavelength of 550 nm ina range from 400 nm to 1800 nm, preferably in a range from 700 nm to1500 nm and most preferably in a range from 900 nm to 1300 nm.

In comparison to the arrangement of FIG. 5A, the privacy mode ofoperation may advantageously achieve increased resilience to theappearance of material flow when the liquid crystal retarder is pressed.

By way of comparison with the present embodiments, the performance ofretarders between parallel polarisers when arranged in series will nowbe described. First, the field of view of a homogeneously aligned liquidcrystal retarder 301 will now be described for two different drivevoltages.

FIG. 19A is a schematic diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable liquid crystalretarder 390; FIG. 19B is a schematic graph illustrating the variationof output transmission with polar direction for transmitted light raysin FIG. 19A for a first applied voltage; and FIG. 19C is a schematicgraph illustrating the variation of output transmission with polardirection for transmitted light rays in FIG. 19A for a second appliedvoltage that is greater than the first applied voltage, comprising thestructure illustrated in TABLE 9. The homogeneously aligned switchableliquid crystal retarder 390 corresponds to the switchable liquid crystalretarder 330 described above and may be applied as the switchable liquidcrystal retarder in any of the devices disclosed herein.

FIG. 19D is a schematic diagram illustrating in perspective side view apassive C-plate retarder 392 arranged between parallel polarisers; andFIG. 19E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIG.19D, comprising the structure illustrated in TABLE 9. The passiveC-plate retarder 392 corresponds to the passive compensation retarder330 and may be applied as the at least one passive compensation retarderin any of the devices disclosed herein.

TABLE 9 Passive compensation retarder(s) Active LC retarder Δn · d/Cenrtal Alignment Pretilt/ Δn · d/ Voltage/ FIG. Type nm polariser?layers deg nm Δϵ V 19A & 19B — — — Homogeneous 1 900 +15 2.4 19CHomogeneous 20.0 19D & 19E Negative C −700 — — — — — — 20A & 20BNegative C −700 Yes Homogeneous 1 900 +15 2.4 20C Homogeneous 20.0 21A &21B Negative C −700 No Homogeneous 1 900 +15 2.4 21C Homogeneous 20.0

FIG. 20A is a schematic diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable liquid crystalretarder 390 arranged between parallel polarisers 394, 396 in serieswith a field-of-view control passive retarder comprising a C-plateretarder 392 arranged between parallel polarisers 396, 398; FIG. 20B isa schematic graph illustrating the variation of output transmission withpolar direction for transmitted light rays in FIG. 20A for a firstapplied voltage; FIG. 20C is a schematic graph illustrating thevariation of output transmission with polar direction for transmittedlight rays in FIG. 20A for a second applied voltage that is greater thanthe first applied voltage, comprising the structure illustrated in TABLE9.

FIG. 21A is a schematic diagram illustrating in perspective side view anarrangement of a homogeneously aligned switchable liquid crystalretarder 301 in series with a C-plate compensation retarder 330 whereinthe homogeneously aligned switchable liquid crystal material 712 andC-plate compensation retarder 330 are arranged between a single pair ofparallel polarisers; FIG. 21B is a schematic graph illustrating thevariation of output transmission with polar direction for transmittedlight rays in FIG. 21A for a first applied voltage; and FIG. 21C is aschematic graph illustrating the variation of output transmission withpolar direction for transmitted light rays in FIG. 21A for a secondapplied voltage that is greater than the first applied voltage,comprising the structure illustrated in TABLE 9.

Unexpectedly, the optimum conditions for maximum field-of-view operationis provided by equal and opposite net retardation of the compensationretarder 330 in comparison to the switchable liquid crystal retarder 301in its undriven state. An ideal compensation retarder 330 and switchableliquid crystal retarder 301 may achieve (i) no modification of the wideangle mode performance from the input light and (ii) optimal reductionof lateral viewing angle for off-axis positions for all elevations whenarranged to provide a narrow angle state. This teaching may be appliedto all the display devices disclosed herein.

It may be desirable to increase the reduction of luminance for off-axisviewing positions. In particular it would be desirable to provideincreased privacy reduction in a liquid crystal display with a wideangle backlight.

FIG. 22A is a schematic diagram illustrating in perspective side view(and noting the reversed view in which the z-axis along which outputlight is directed is downwards) an arrangement of a switchable retarderin a privacy mode of operation, comprising: a first switchablecompensated retarder 300A (in this case, a negative C-plate passivecompensation retarder 330A and homeotropically aligned switchable liquidcrystal retarder 301A, but this is merely an example and may be replacedby any of the other arrangements of plural retarders disclosed herein)arranged between the output display polariser 218 and an additionalpolariser 318A; and a further switchable compensated retarder 300B (inthis case, a negative C-plate passive compensation retarder 330B andhomeotropically aligned switchable liquid crystal retarder 301B, butthis is merely an example and may be replaced by any of the otherarrangements of plural retarders disclosed herein) arranged between thefirst-mentioned additional polariser 318A and a further additionalpolariser 318B with electric vector transmission direction 319B.

As an alternative, the first-mentioned additional polariser 318A may bearranged on the input side of the input display polariser 210, in whichcase the further additional polariser 318B may be arranged on the inputside of the input display polariser 210 between the first-mentionedadditional polariser 318A and the backlight 20, and the furtherswitchable compensated retarder 300B may be arranged between the furtheradditional polariser 318B and the first-mentioned additional polariser318A.

In both of these alternatives, each of the first plural retarders 300Aand the further plural retarders 300B are arranged between a respectivepair of polarisers and so have an effect similar to that of thecorresponding structure in the devices described above.

The pretilt directions 307A, 309AA of the alignment layers of thefurther switchable liquid crystal retarder 301A may have a component inthe plane of the liquid crystal layer that is aligned parallel orantiparallel or orthogonal to the pretilt directions of the alignmentlayers 307B, 309AB of the first switchable liquid crystal retarder 301B.In a wide angle mode of operation, both switchable liquid crystalretarders 301A, 301B are driven to provide a wide viewing angle. In aprivacy mode of operation, switchable liquid crystal retarders 301B,301A may cooperate to advantageously achieve increased luminancereduction and thus improved privacy in a single axis.

The retardation provided by the first switchable liquid crystal retarder301B and further liquid crystal retarders 301A may be different. Theswitchable liquid crystal retarder 301B and further switchable liquidcrystal retarder 301A may be driven by a common voltage and the liquidcrystal material 408B in the first switchable liquid crystal retarder301B may be different to the liquid crystal material 408A in the furtherswitchable liquid crystal retarder 301A. Chromatic variation of thepolar luminance profiles illustrated elsewhere herein may be reduced, sothat advantageously off-axis color appearance is improved.

Alternatively, switchable liquid crystal retarders 301B, 301A may haveorthogonal alignments so that reduced luminance is achieved in bothhorizontal and vertical directions, to advantageously achieve landscapeand portrait privacy operation.

Alternatively, the layers 301A, 301B may be provided with differentdrive voltages. Advantageously increased control of roll-off ofluminance profile may be achieved or switching between landscape andprivacy operation may be provided.

The retardance control layer 330B may comprise a passive compensationretarder 330A arranged between the first additional polariser 318A andthe further additional polariser 318B. More generally, the switchableliquid crystal retarder 301A may be omitted and a fixed luminancereduction may be provided by passive compensation retarders 330A. Forexample, luminance reduction in viewing quadrants may be provided bymeans of layer 330A alone. Advantageously increased area of the polarregion for luminance reduction may be achieved. Further, backlights thathave a wider angle of illumination output than collimated backlights maybe provided, increasing the visibility of the display in wide angle modeof operation.

FIG. 22B is a schematic diagram illustrating in perspective side view anarrangement of first switchable compensated retarder arranged on theinput of a liquid crystal display and a second switchable compensatedretarder arranged on the output of a liquid crystal display.

The first-mentioned additional polariser 318A is arranged on the inputside of the input display polariser 210 between the input displaypolariser 210 and the backlight 20, and the display device furthercomprises: a further additional polariser 318B arranged on the outputside of the output display polariser 218; and further retarders 301B,330B arranged between the further additional polariser 318B and theoutput display polariser 218. The further retarders comprise a furtherswitchable liquid crystal retarder 301B comprising a layer of liquidcrystal material 414B and electrodes 413B, 415B on opposite sides of thelayer of liquid crystal material 414B, the layer of liquid crystalmaterial 414B being switchable between two orientation states by meansof a voltage being applied across the electrodes 413B, 415B.

FIG. 22C is a schematic diagram illustrating in side perspective view aview angle control optical element comprising a first passivecompensation retarder, a first switchable liquid crystal retarder, afirst control polariser 250, a second passive compensation retarder, asecond switchable liquid crystal retarder and a second control polariser250. Such an element may achieve similar performance to the arrangementof FIG. 22B when provided for display device 100 comprising spatiallight modulator 48.

It may be desirable to provide both entertainment and night-time modesof operation in an automotive vehicle.

FIG. 22D is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display such as that illustratedin FIG. 22B arranged within the vehicle cabin 602 for day-time and/orsharing modes of operation; and FIG. 22E is a schematic diagramillustrating in side view an automotive vehicle with a switchabledirectional display arranged within the vehicle cabin 602 for day-timeand/or sharing modes of operation. Light cone 630, 632 is provided witha wide angular field of view and thus the display is advantageouslyvisible by multiple occupants.

FIG. 22F is a schematic diagram illustrating in top view an automotivevehicle with a switchable directional display such as that illustratedin FIG. 22B arranged within the vehicle cabin 602 for night-time and/orentertainment modes of operation; FIG. 22G is a schematic diagramillustrating in side view an automotive vehicle with a switchabledirectional display arranged within the vehicle cabin 602 for night-timeand/or entertainment modes of operation. Light cone 634, 636 is providedwith a narrow angular field of view and thus the display isadvantageously visible only by a single occupant. Advantageously straylight for night-time operation is reduced, increasing driver safety.Further, reflections of the display from windscreen 601 are reduced,minimising distraction to the driver 604.

It would be desirable to provide a reduced field of view for light conesthat are provided by wide angle illumination backlights and emissivespatial light modulators and at low cost.

FIG. 23A is a schematic diagram illustrating in perspective side view anarrangement of a reflective additional polariser 318A and a passiveretarder 270 arranged on the input of a spatial light modulator 48. Onthe output of the spatial light modulator 48, there are plural retarders300 similar to those in the device of FIG. 22B. In comparison to thearrangement of FIG. 22B, passive retarder 270 is provided in place ofthe rear compensated switchable liquid crystal retarder 300A.Advantageously the cost and thickness is reduced, while achieving lowoff-axis illumination in privacy mode of operation and acceptableviewing angle in wide mode of operation.

FIG. 23B is a schematic diagram illustrating in side perspective view aview angle control optical element comprising a passive retarder 270, afirst control polariser 250A, a passive compensation retarder 330, aswitchable liquid crystal retarder 301 and a second control polariser250B. This arranged on front of a spatial light modulator 48 to providea display device.

Various passive retarders 270 will now be described, any of which may beapplied in any of the above devices.

FIG. 24A is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarder 270 comprising a negative O-plateretarder 272A tilted in a plane orthogonal to the display polariserelectric vector transmission direction and a negative C-plate retarder272B and arranged to provide field-of-view modification of a displaydevice; and FIG. 24B is a schematic graph illustrating the variation ofoutput transmission with polar direction for transmitted light rays inthe passive retarder of FIG. 24A, comprising the structure illustratedin TABLE 10.

TABLE 10 Passive retarder Out of plane In plane Δn.d FIGURES Layer Typeangle/° angle/° mm 24A & 24B 272A Negative O 65 90 −550 272B Positive C90 0 +500

The passive retarder 270 thus comprises a passive retarder 272A that isa negative O-plate which has an optical axis with a component in theplane of the passive retarder 272A and a component perpendicular to theplane of the passive retarder 272A. Further the component in the planeof the passive retarder extends at 90°, with respect to an electricvector transmission direction that is parallel to the electric vectortransmission 219 of the display polariser 218. The passive retarder 272Bcomprises a passive retarder having an optical axis perpendicular to theplane of the passive retarder.

Advantageously luminance may be reduced for lateral viewing directions.A mobile display may be comfortably rotated about a horizontal axiswhile achieving privacy for off-axis snoopers in a lateral direction.

FIG. 24C is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarder 270 comprising crossed A-plates anda positive O-plate; and FIG. 24D is a schematic graph illustrating thevariation of output transmission with polar direction for transmittedlight rays in the passive retarder of FIG. 24C, comprising the structureillustrated in TABLE 11.

TABLE 11 Passive retarder Out of plane In plane Δn.d FIGURES Layer Typeangle/° angle/° mm 24C & 24D 272A Positive A 0 45 +500 272B Positive A 0135 +500 272C Positive O 65 90 +550

The passive retarder 270 thus comprises passive retarders 272A, 272Bthat are crossed A-plates and retarder 272C which has an optical axiswith a component in the plane of the passive retarder 272C and acomponent perpendicular to the plane of the passive retarder 272C. Thecomponent in the plane of the passive retarder extends at 90°, withrespect to an electric vector transmission direction that is parallel tothe electric vector transmission 219 of the display polariser 218.Advantageously luminance may be reduced for lateral viewing directions.A mobile display may be comfortably rotated about a horizontal axiswhile achieving privacy for off-axis snoopers in a lateral direction.

It may be desirable to provide reduction of luminance in both lateraland elevation directions.

FIG. 24E is a schematic diagram illustrating in side perspective view anoptical stack of a passive retarders 272A-D comprising two pairs ofcrossed A-plates; and FIG. 24F is a schematic graph illustrating thevariation of output transmission with polar direction for transmittedlight rays in the passive retarder of FIG. 24E, comprising the structureillustrated in TABLE 12.

TABLE 12 Passive control retarder Out of plane In plane Δn.d FIGURESLayer Type angle/° angle/° mm 24E, 24F 272A Positive A 0 45 700 272B 90272C 0 272D 135

The retarder 270 thus comprises a pair of passive retarders 272A, 272Dwhich have optical axes in the plane of the retarders that are crossed.The pair of retarders each comprise plural A-plates having respectiveoptical axes aligned at different angles from each other. The pair ofpassive retarders 272B, 272C have optical axes that each extend at 90°and 0°, respectively, with respect to an electric vector transmissiondirection that is parallel to the electric vector transmission 211 ofthe display polariser 210.

The pair of passive retarders 272A, 272D have optical axes that extendat 45° and at 135°, respectively, with respect to an electric vectortransmission direction 211 that is parallel to the electric vectortransmission of the display polariser 218.

The display further comprises an additional pair of passive retarders272B, 272C disposed between the first-mentioned pair of passiveretarders 272A, 272D and which have optical axes in the plane of theretarders that are crossed. The additional pair of passive retarders272B, 272C have optical axes that each extend at 0° and at 90°,respectively, with respect to an electric vector transmission direction211, 317 that is parallel to the electric vector transmission of thedisplay polariser 210, 316.

The retardance of each A-plate for light of a wavelength of 550 nm maybe in a range from 600 nm to 850 nm, preferably in a range from 650 nmto 730 nm, and most preferably in a range from 670 nm to 710 nm. Thecolor change of absorbed light from a central viewing location to anoff-axis viewing location may be advantageously reduced.

In further illustrative embodiments, preferably the angle 273A is atleast 40° and at most 50°, more preferably at least 42.5° and at most47.5° and most preferably at least 44° and at most 46°. Preferably theangle 273D is at least 130° and at most 140°, more preferably at least132.5° and at most 137.5° and most preferably at least 134° and at most136°.

In further illustrative embodiments, the inner retarder pair 272B, 272Cmay have looser tolerances than the outer retarder pair 272A, 272D.Preferably the angle 273B is at least −10° and at most 10°, morepreferably at least −5° and at most 5° and most preferably at least −2°and at most 2°. Preferably the angle 273C is at least 80° and at most100°, more preferably at least 85° and at most 95° and most preferablyat least 88° and at most 92°.

The present embodiment provides a transmission profile that has somerotational symmetry. Advantageously a privacy display may be providedwith reduced visibility of image from a wide field of view for lateralor elevated viewing positions of a snooper. Further, such an arrangementmay be used to achieve enhanced privacy operation for landscape andportrait operation of a mobile display. Such an arrangement may beprovided in a vehicle to reduce stray light to off-axis passengers, andalso to reduce light falling on windscreen and other glass surfaces inthe vehicle.

It would be desirable to provide improved image appearance by means ofadding camouflage to the private image seen by the snooper 47 in privacymode of operation.

FIG. 25A is a schematic diagram illustrating in perspective side view anarrangement of a switchable retarder in a privacy mode of operationcomprising a negative C-plate passive compensation retarder andhomeotropically aligned switchable liquid crystal retarder furthercomprising a patterned electrode 415 layer. Thus the electrodes 415 a,415 b, 415 c are patterned to provide at least two pattern regions.

At least one of the electrodes 413, 415 may be patterned, in thisexample electrode 415 is patterned with regions 415 a, 415 b, 415 c anddriven by respective voltage drivers 350 a, 350 b, 350 c with voltagesVa, Vb, Vc. Gaps 417 may be provided between the electrode regions 415a, 415 b, 415 c. The tilt of the material 414 a, 414 b, 414 c may thusbe adjusted independently to reveal a camouflage pattern with differentluminance levels for off-axis viewing.

Thus the switchable liquid crystal retarder arranged between the outputdisplay polariser 218 and the additional absorbing polariser 318 iscontrolled by means of addressing electrodes 415 a, 415 b, 415 c anduniform electrode 413. The addressing electrodes may be patterned toprovide at least two pattern regions comprising electrode 415 a and gap417.

FIG. 25B is a schematic diagram illustrating in perspective front viewillumination of a primary viewer and a snooper by a camouflagedluminance controlled privacy display. Display device 100 may have darkimage data 601 and white background data 603 that is visible to theprimary viewer 45 in viewing window 26 p. By way of comparison snooper47 may see the camouflaged image as illustrated in FIG. 25C which is aschematic diagram illustrating in perspective side view illumination ofa snooper by a camouflaged luminance controlled privacy display. Thus inwhite background regions 603, a camouflage structure may be providedthat has mixed luminance of the white region 603. The pattern regions ofthe electrodes 415 a, 415 b, 415 c are thus camouflage patterns. Atleast one of the pattern regions is individually addressable and isarranged to operate in a privacy mode of operation.

The pattern regions may be arranged to provide camouflage for multiplespatial frequencies by means of control of which patterns are providedduring privacy mode of operation. In an illustrative example, apresentation may be provided with 20 mm high text. A camouflage patternwith similar pattern size may be provided with a first control of anelectrode pattern. In a second example a photo may be provided withlarge area content that is most visible to a snooper 47. The spatialfrequency of the camouflage pattern may be reduced to hide the largerarea structures, by combining first and second electrode regions toprovide the voltage and achieve a resultant lower spatial frequencypattern.

Advantageously a controllable camouflage structure may be provided bymeans of adjustment of the voltages Va, Vb, Vc across the layer 892.Substantially no visibility of the camouflage structure may be seen forhead-on operation. Further the camouflage image may be removed byproviding Va, Vb and Vc to be the same.

It would be desirable to provide off-axis luminance to snoopers withluminance that is for example less than 1%. Directional backlights thatprovide low off-axis luminance may be used together with the compensatedswitchable liquid crystal retarders of the present embodiments will nowbe described. Directional backlights will now be further described.

Similar patterning may be applied in any of the devices describedherein.

It would be desirable to provide further reduction of off-axis luminanceby means of directional illumination from the spatial light modulator48. Directional illumination of the spatial light modulator 48 bydirectional backlights 20 will now be described.

FIG. 26A is a schematic diagram illustrating in front perspective view adirectional backlight 20, and FIG. 26B is a schematic diagramillustrating in front perspective view a non-directional backlight 20,either of which may be applied in any of the devices described herein.Thus a directional backlight 20 as shown in FIG. 26A provides a narrowcone 450, whereas a non-directional backlight 20 as shown in FIG. 26Bprovides a wide angular distribution cone 452 of light output rays.

FIG. 26C is a schematic graph illustrating variation with luminance withlateral viewing angle for various different backlight arrangements. Thegraph of FIG. 26C may be a cross section through the polar field-of-viewprofiles described herein.

A Lambertian backlight has a luminance profile 846 that is independentof viewing angle.

A typical wide angle backlight has a roll-off at higher angles such thatthe full width half maximum 866 of relative luminance may be greaterthan 40°, preferably greater than 60° and most preferably greater than80°. Further the relative luminance 864 at +/−45°, is preferably greaterthan 7.5%, more preferably greater than 10% and most preferably greaterthan 20%.

By way of comparison a directional backlight 20 has a roll-off at higherangles such that the full width half maximum 862 of relative luminancemay be less than 60°, preferably less than 40° and most preferably lessthan 20°. Further the backlight 20 may provide a luminance at polarangles to the normal to the spatial light modulator 48 greater than 45degrees that is at most 33% of the luminance along the normal to thespatial light modulator 48, preferably at most 20% of the luminancealong the normal to the spatial light modulator 48, and most preferablyat most 10% of the luminance along the normal to the spatial lightmodulator 48.

Scatter and diffraction in the spatial light modulator 48 may degradeprivacy mode operation when the switchable retarder 300 is arrangedbetween the input display polariser 210 and additional polariser 318.The luminance at polar angles to the normal to the spatial lightmodulator greater than 45 degrees may be increased in arrangementswherein the switchable retarder 300 is arranged between the outputdisplay polariser 218 and additional polariser 318 in comparison toarrangements wherein the switchable retarder 300 is arranged between theinput display polariser 210 and additional polariser 318.

Advantageously lower off-axis luminance may be achieved for thearrangement of FIG. 1A in comparison to FIG. 2A for the same backlight20.

In an illustrative embodiment of FIG. 1A, the luminance at polar anglesto the normal to the spatial light modulator 48 greater than 45 degreesmay be at most 18% whereas in an illustrative embodiment of FIG. 2A, theluminance at polar angles to the normal to the spatial light modulator48 greater than 45 degrees may be at most 10%. Advantageously theembodiment of FIG. 1A may provide a wider viewing freedom in wide anglemode of operation while achieving similar viewing freedom to theembodiment of FIG. 2A in privacy mode of operation.

Such luminance profiles may be provided by the directional backlights 20described below or may also be provided by wide angle backlights incombination with further additional polariser 318B and passive retarders270 or additional compensated switching liquid crystal retarder 300B.

FIG. 27A is a schematic diagram illustrating in side view a switchabledirectional display apparatus 100 comprising a switchable liquid crystalretarder 300 and backlight 20. The backlight 20 of FIG. 27A may beapplied in any of the devices described herein and which comprises animaging waveguide 1 illuminated by a light source array 15 through aninput end 2. FIG. 27B which is a schematic diagram illustrating in rearperspective view operation of the imaging waveguide 1 of FIG. 27A in anarrow angle mode of operation.

The imaging waveguides 1 is of the type described in U.S. Pat. No.9,519,153, which is herein incorporated by reference in its entirety.The waveguide 1 has an input end 2 extending in a lateral directionalong the waveguide 1. An array of light sources 15 are disposed alongthe input end 2 and input light into the waveguide 1.

The waveguide 1 also has opposed first and second guide surfaces 6, 8extending across the waveguide 1 from the input end 2 to a reflectiveend 4 for guiding light input at the input end 2 forwards and back alongthe waveguide 1. The second guide surface 8 has a plurality of lightextraction features 12 facing the reflective end 4 and arranged todeflect at least some of the light guided back through the waveguide 1from the reflective end 4 from different input positions across theinput end 2 in different directions through the first guide surface 6that are dependent on the input position.

In operation, light rays are directed from light source array 15 throughan input end and are guided between first and second guiding surfaces 6,8 without loss to a reflective end 4. Reflected rays are incident ontofacets 12 and output by reflection as light rays 230 or transmitted aslight rays 232. Transmitted light rays 232 are directed back through thewaveguide 1 by facets 803, 805 of rear reflector 800. Operation of rearreflectors are described further in U.S. Pat. No. 10,054,732, which isherein incorporated by reference in its entirety.

As illustrated in FIG. 27B, optical power of the curved reflective end 4and facets 12 provide an optical window 26 that is transmitted throughthe spatial light modulator 48 and has an axis 197 that is typicallyaligned to the optical axis 199 of the waveguide 1. Similar opticalwindow 26 is provided by transmitted light rays 232 that are reflectedby the rear reflector 800.

FIG. 27C is a schematic graph illustrating field-of-view luminance plotof the output of FIG. 27B when used in a display apparatus with noswitchable liquid crystal retarder.

Thus for off-axis viewing positions observed by snoopers 47 may havereduced luminance, for example between 1% and 3% of the central peakluminance at an elevation of 0 degrees and lateral angle of +/−45degrees. Further reduction of off-axis luminance is achieved by theplural retarders 301, 330 of the present embodiments.

Another type of directional backlight with low off-axis luminance willnow be described.

FIG. 28A is a schematic diagram illustrating in side view a switchabledirectional display apparatus comprising a backlight 20 including aswitchable collimating waveguide 901 and a switchable liquid crystalretarder 300 and additional polariser 318. The backlight 20 of FIG. 28Amay be applied in any of the devices described herein and is arranged asfollows.

The waveguide 901 has an input end 902 extending in a lateral directionalong the waveguide 901. An array of light sources 915 are disposedalong the input end 902 and input light into the waveguide 1. Thewaveguide 901 also has opposed first and second guide surfaces 906, 908extending across the waveguide 1 from the input end 2 to a reflectiveend 4 for guiding light input at the input end 2 forwards and back alongthe waveguide 1. In operation, light is guided between the first andsecond guiding surface 906, 908.

The first guiding surface 906 may be provided with a lenticularstructure 904 comprising a plurality of elongate lenticular elements 905and the second guiding surface 908 may be provided with prismaticstructures 912 which are inclined and act as light extraction features.The plurality of elongate lenticular elements 905 of the lenticularstructure 904 and the plurality of inclined light extraction featuresdeflect input light guided through the waveguide 901 to exit through thefirst guide surface 906.

A rear reflector 903 that may be a planar reflector is provided todirect light that is transmitted through the surface 908 back throughthe waveguide 901.

Output light rays that are incident on both the prismatic structures 912and lenticular elements 905 of the lenticular structure 904 are outputat angles close to grazing incidence to the surface 906. A prismaticturning film 926 comprising facets 927 is arranged to redirect outputlight rays 234 by total internal reflection through the spatial lightmodulator 48 and compensated switchable liquid crystal retarder 300.

FIG. 28B is a schematic diagram illustrating in top view output of thecollimating waveguide 901. Prismatic structures 912 are arranged toprovide light at angles of incidence onto the lenticular structure 904that are below the critical angle and thus may escape. On incidence atthe edges of a lenticular surface, the inclination of the surfaceprovides a light deflection for escaping rays and provides a collimatingeffect. Light ray 234 may be provided by light rays 188 a-c and lightrays 189 a-c, with incidence on locations 185 of the lenticularstructure 904 of the collimated waveguide 901.

FIG. 28C is a schematic graph illustrating an iso-luminancefield-of-view polar plot for the display apparatus of FIG. 28A. Thus anarrow output light cone may be provided, with size determined by thestructures 904, 912 and the turning film 926.

Advantageously in regions in which snoopers may be located with lateralangles of 45 degrees or greater for example, the luminance of outputfrom the display is small, typically less than 2%. It would be desirableto achieve further reduction of output luminance. Such further reductionis provided by the compensated switchable liquid crystal retarder 300and additional polariser 318 as illustrated in FIG. 28A. Advantageouslya high performance privacy display with low off-axis luminance may beprovided over a wide field of view.

Directional backlights such as the types described in FIG. 27A and FIG.28A together with the plural retarders 301, 330 of the presentembodiments may achieve off-axis luminance of less than 1.5%, preferablyless than 0.75% and most preferably less than 0.5% may be achieved fortypical snooper 47 locations. Further, high on-axis luminance anduniformity may be provided for the primary user 45. Advantageously ahigh performance privacy display with low off-axis luminance may beprovided over a wide field of view, that may be switched to a wide anglemode by means of control of the switchable retarder 301 by means ofcontrol system 352 illustrated in FIG. 1A.

The operation of retarder layers between parallel polarisers foroff-axis illumination will now be described further. In the variousdevices described above, retarders are arranged between a pair ofpolarisers (typically the additional polariser 318 and one of the inputpolariser 210 and output polariser 218) in various differentconfigurations. In each case, the retarders are configured so that theynot affect the luminance of light passing through the pair of polarisersand the plural retarders along an axis along a normal to the plane ofthe retarders but they do reduce the luminance of light passing throughthe pair of polarisers and the plural retarders along an axis inclinedto a normal to the plane of the retarders, at least in one of theswitchable states of the compensated switchable retarder 300. There willnow be given a description of this effect in more detail, the principlesof which may be applied in general to all of the devices describedabove.

FIG. 29A is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light. Correction retarder630 may comprise birefringent material, represented by refractive indexellipsoid 632 with optical axis direction 634 at 0 degrees to thex-axis, and have a thickness 631. Normal light rays 636 propagate sothat the path length in the material is the same as the thickness 631.Light rays 637 are in the y-z plane have an increased path length;however the birefringence of the material is substantially the same asthe rays 636. By way of comparison light rays 638 that are in the x-zplane have an increased path length in the birefringent material andfurther the birefringence is different to the normal ray 636.

The retardance of the retarder 630 is thus dependent on the angle ofincidence of the respective ray, and also the plane of incidence, thatis rays 638 in the x-z will have a retardance different from the normalrays 636 and the rays 637 in the y-z plane.

The interaction of polarized light with the retarder 630 will now bedescribed. To distinguish from the first and second polarizationcomponents during operation in a directional backlight 101, thefollowing explanation will refer to third and fourth polarizationcomponents.

FIG. 29B is a schematic diagram illustrating in perspective viewillumination of a retarder layer by off-axis light of a third linearpolarization state at 90 degrees to the x-axis and FIG. 29C is aschematic diagram illustrating in perspective view illumination of aretarder layer by off-axis light of a fourth linear polarization stateat 0 degrees to the x-axis. In such arrangements, the incident linearpolarization states are aligned to the optical axes of the birefringentmaterial, represented by ellipse 632. Consequently, no phase differencebetween the third and fourth orthogonal polarization components isprovided, and there is no resultant change of the polarization state ofthe linearly polarized input for each ray 636, 637, 638. Thus, theretarder 630 introduces no phase shift to polarisation components oflight passed by the polariser on the input side of the retarder 630along an axis along a normal to the plane of the retarder 630.Accordingly, the retarder 630 does not affect the luminance of lightpassing through the retarder 630 and polarisers (not shown) on each sideof the retarder 630. Although FIGS. 29A-C relate specifically to theretarder 630 that is passive, a similar effect is achieved by aswitchable liquid crystal retarder and by plural retarders in thedevices described above.

FIG. 29D is a schematic diagram illustrating in perspective viewillumination of a retarder 630 layer by off-axis light of a linearpolarization state at 45 degrees. The linear polarization state may beresolved into third and fourth polarization components that arerespectively orthogonal and parallel to optical axis 634 direction. Theretarder thickness 631 and material retardance represented by refractiveindex ellipsoid 632 may provide a net effect of relatively shifting thephase of the third and fourth polarization components incident thereonin a normal direction represented by ray 636 by half a wavelength, for adesign wavelength. The design wavelength may for example be in the rangeof 500 to 550 nm.

At the design wavelength and for light propagating normally along ray636 then the output polarization may be rotated by 90 degrees to alinear polarization state 640 at −45 degrees. Light propagating alongray 637 may see a phase difference that is similar but not identical tothe phase difference along ray 637 due to the change in thickness, andthus an elliptical polarization state 639 may be output which may have amajor axis similar to the linear polarization axis of the output lightfor ray 636.

By way of contrast, the phase difference for the incident linearpolarization state along ray 638 may be significantly different, inparticular a lower phase difference may be provided. Such phasedifference may provide an output polarization state 644 that issubstantially circular at a given inclination angle 642. Thus, theretarder 630 introduces a phase shift to polarisation components oflight passed by the polariser on the input side of the retarder 630along an axis corresponding to ray 638 that is inclined to a normal tothe plane of the retarder 630. Although FIG. 29D relates to the retarder630 that is passive, a similar effect is achieved by a switchable liquidcrystal retarder, and in the plural retarders described above, in aswitchable state of the switchable liquid crystal retarder correspondingto the privacy mode.

To illustrate the off-axis behavior of retarder stacks, the angularluminance control of C-plates 308A, 308B between an additional polariser318 and output display polariser 218 will now be described for variousoff-axis illumination arrangements with reference to the operation of aC-plate 560 between the parallel polarisers 500, 210.

FIG. 30A is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with apositive elevation. Incident linear polarisation component 704 isincident onto the birefringent material 632 of the retarder 560 that isa C-plate with optical axis direction 507 that is perpendicular to theplane of the retarder 560. Polarisation component 704 sees no net phasedifference on transmission through the liquid crystal molecule and sothe output polarisation component is the same as component 704. Thus amaximum transmission is seen through the polariser 210. Thus theretarder comprises a retarder 560 having an optical axis 561perpendicular to the plane of the retarder 560, that is the x-y plane.The retarder 560 having an optical axis perpendicular to the plane ofthe retarder comprises a C-plate.

FIG. 30B is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with anegative lateral angle. As with the arrangement of FIG. 30A,polarisation state 704 sees no net phase difference and is transmittedwith maximum luminance. Thus, the retarder 560 introduces no phase shiftto polarisation components of light passed by the polariser on the inputside of the retarder 560 along an axis along a normal to the plane ofthe retarder 560. Accordingly, the retarder 560 does not affect theluminance of light passing through the retarder 560 and polarisers (notshown) on each side of the retarder 560. Although FIGS. 29A-C relatespecifically to the retarder 560 that is passive, a similar effect isachieved by a switchable liquid crystal retarder and by plural retardersin the devices described above.

FIG. 30C is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with apositive elevation and negative lateral angle. In comparison to thearrangement of FIGS. 30A-B, the polarisation state 704 resolves ontoeigenstates 703, 705 with respect to the birefringent material 632providing a net phase difference on transmission through the retarder560. The resultant elliptical polarisation component 656 is transmittedthrough polariser 210 with reduced luminance in comparison to the raysillustrated in FIGS. 30A-B.

FIG. 30D is a schematic diagram illustrating in perspective viewillumination of a C-plate layer by off-axis polarised light with apositive elevation and positive lateral angle. In a similar manner toFIG. 30C, the polarisation component 704 is resolved into eigenstates703, 705 that undergo a net phase difference, and ellipticalpolarisation component 660 is provided, which after transmission throughthe polariser reduces the luminance of the respective off-axis ray.Thus, the retarder 560 introduces a phase shift to polarisationcomponents of light passed by the polariser on the input side of theretarder 560 along an axis that is inclined to a normal to the plane ofthe retarder 560. Although FIG. 29D relates to the retarder 560 that ispassive, a similar effect is achieved by a switchable liquid crystalretarder, and in the plural retarders described above, in a switchablestate of the switchable liquid crystal retarder corresponding to theprivacy mode

FIG. 30E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.30A-D. Thus, the C-plate may provide luminance reduction in polarquadrants. In combination with switchable liquid crystal retarder 301described elsewhere herein, (i) removal of luminance reduction of theC-plate may be provided in a first wide angle state of operation, and(ii) extended polar region for luminance reduction may be achieved in asecond privacy state of operation.

To illustrate the off-axis behavior of retarder stacks, the angularluminance control of crossed A-plates 308A, 308B between an additionalpolariser 318 and output display polariser 218 will now be described forvarious off-axis illumination arrangements.

FIG. 31A is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation. Linear polariser 218 with electricvector transmission direction 219 is used to provide a linearpolarisation state 704 that is parallel to the lateral direction ontofirst A-plate 308A of the crossed A-plates 308A, 308B. The optical axisdirection 309A is inclined at +45 degrees to the lateral direction. Theretardance of the retarder 308A for the off-axis angle θ₁ in thepositive elevation direction provides a resultant polarisation component650 that is generally elliptical on output. Polarisation component 650is incident onto the second A-plate 308B of the crossed A-plates 308A,308B that has an optical axis direction 309B that is orthogonal to theoptical axis direction 309A of the first A-plate 308A. In the plane ofincidence of FIG. 31A, the retardance of the second A-plate 308B for theoff-axis angle θ₁ is equal and opposite to the retardance of the firstA-plate 308A. Thus a net zero retardation is provided for the incidentpolarisation component 704 and the output polarisation component is thesame as the input polarisation component 704.

The output polarisation component is aligned to the electric vectortransmission direction of the additional polariser 318, and thus istransmitted efficiently. Advantageously substantially no losses areprovided for light rays that have zero lateral angle angular componentso that full transmission efficiency is achieved.

FIG. 31B is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a negative lateral angle. Thus input polarisation componentis converted by the first A-plate 308A to an intermediate polarisationcomponent 652 that is generally an elliptical polarisation state. Thesecond A-plate 308B again provides an equal and opposite retardation tothe first A-plate so that the output polarisation component is the sameas the input polarisation component 704 and light is efficientlytransmitted through the polariser 318.

Thus the retarder comprises a pair of retarders 308A, 308B which haveoptical axes in the plane of the retarders 308A, 308B that are crossed,that is the x-y plane in the present embodiments. The pair of retarders308A, 308B have optical axes 309A, 309B that each extend at 45° withrespect to an electric vector transmission direction that is parallel tothe electric vector transmission of the polariser 318.

Advantageously substantially no losses are provided for light rays thathave zero elevation angular component so that full transmissionefficiency is achieved.

FIG. 31C is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and negative lateral angle.

Polarisation component 704 is converted to an elliptical polarisationcomponent 654 by first A-plate 308A. A resultant elliptical component656 is output from the second A-plate 308B. Elliptical component 656 isanalysed by input polariser 318 with reduced luminance in comparison tothe input luminance of the first polarisation component 704.

FIG. 31D is a schematic diagram illustrating in perspective viewillumination of crossed A-plate retarder layers by off-axis polarisedlight with a positive elevation and positive lateral angle. Polarisationcomponents 658 and 660 are provided by first and second A-plates 308A,308B as net retardance of first and second retarders does not providecompensation.

Thus luminance is reduced for light rays that have non-zero lateralangle and non-zero elevation components. Advantageously display privacycan be increased for snoopers that are arranged in viewing quadrantswhile luminous efficiency for primary display users is not substantiallyreduced.

FIG. 31E is a schematic graph illustrating the variation of outputtransmission with polar direction for transmitted light rays in FIGS.31A-D. In comparison to the arrangement of FIG. 30E, the area ofluminance reduction is increased for off-axis viewing. However, theswitchable liquid crystal retarder 301 may provide reduced uniformity incomparison to the C-plate arrangements for off-axis viewing in the firstwide mode state of operation.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom zero percent to ten percent and corresponds to, but is not limitedto, component values, angles, et cetera. Such relativity between itemsranges between approximately zero percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

1. A display device comprising: a backlight arranged to output light; atransmissive spatial light modulator arranged to receive output lightfrom the backlight; an input display polariser arranged on an input sideof the spatial light modulator; an output display polariser arranged onan output side of the spatial light modulator; a first additionalpolariser arranged on the input side of the spatial light modulatorbetween the input display polariser and the backlight; at least onefirst retarder arranged between the first additional polariser and theinput display polariser, wherein the at least one first retardercomprises: a first switchable liquid crystal retarder comprising a layerof liquid crystal material and two surface alignment layers disposedadjacent to the layer of liquid crystal material and on opposite sidesthereof, each of the surface alignment layers being arranged to providehomogeneous alignment in the adjacent liquid crystal material; a secondadditional polariser arranged on the output side of the spatial lightmodulator; and at least one second retarder arranged between the secondadditional polariser and the output display polariser, wherein the atleast one second retarder comprises: a second switchable liquid crystalretarder comprising a layer of liquid crystal material and two surfacealignment layers disposed adjacent to the layer of liquid crystalmaterial and on opposite sides thereof, each of the surface alignmentlayers being arranged to provide homogeneous alignment in the adjacentliquid crystal material; and wherein at least one of the first andsecond retarders further comprises at least one passive compensationretarder.
 2. A display device according to claim 1, wherein the firstswitchable liquid crystal retarder and the second liquid crystalretarder have retardances that are different.
 3. A display deviceaccording to claim 1, wherein the alignment layers of the firstswitchable liquid crystal retarder have pretilts having first pretiltdirections with a component in the plane of the adjacent liquid crystallayer and the alignment layers of the first switchable liquid crystalretarder have pretilts having second pretilt directions with a componentin the plane of the adjacent liquid crystal layer, the component of thefirst pretilt directions being aligned parallel or antiparallel ororthogonal to the component of the second pretilt directions.
 4. Adisplay device according to claim 1, wherein the input display polariserand the first additional polariser have electric vector transmissiondirections that are parallel to each other, and the output displaypolariser and the second additional polariser have electric vectortransmission directions that are parallel to each other.
 5. A displaydevice according to claim 1, wherein the layer of liquid crystalmaterial of the first and second switchable liquid crystal retarderseach comprise a liquid crystal material with a positive dielectricanisotropy.
 6. A display device according to claim 5, wherein the layerof liquid crystal material of at least one of the first and secondswitchable liquid crystal retarders has a retardance for light of awavelength of 550 nm in a range from 500 nm to 1000 nm.
 7. A displaydevice according to claim 5, wherein the layer of liquid crystalmaterial of at least one of the first and second switchable liquidcrystal retarders has a retardance for light of a wavelength of 550 nmin a range from 600 nm to 850 nm.
 8. A display device according to claim5, wherein the layer of liquid crystal material of at least one of thefirst and second switchable liquid crystal retarders has a retardancefor light of a wavelength of 550 nm in a range from 700 nm to 800 nm. 9.A display device according to claim 1, wherein the at least one passivecompensation retarder of the at least one first and second retarderscomprises a passive compensation retarder having its optical axisperpendicular to the plane of the passive compensation retarder.
 10. Adisplay device according to claim 9, wherein the at least one passivecompensation retarder of the at least one first and second retarders hasa retardance for light of a wavelength of 550 nm in a range from −300 nmto −700 nm.
 11. A display device according to claim 9, wherein the atleast one passive compensation retarder of the at least one first andsecond retarders has a retardance for light of a wavelength of 550 nm ina range from −350 nm to −600 nm.
 12. A display device according to claim9, wherein the at least one passive compensation retarder of the atleast one first and second retarders has a retardance for light of awavelength of 550 nm in a range from −400 nm to −500 nm.
 13. A displaydevice according to claim 1, wherein the at least one passivecompensation retarder of the at least one first and second retarderscomprises a pair of passive compensation retarders which have opticalaxes in the plane of the retarders that are crossed.
 14. A displaydevice according to claim 13, wherein each passive compensation retarderof the pair of passive compensation retarders of at least one of thefirst and second retarders has a retardance for light of a wavelength of550 nm in a range from 300 nm to 800 nm.
 15. A display device accordingclaim 13, wherein each passive compensation retarder of the pair ofpassive compensation retarders has a retardance for light of awavelength of 550 nm in a range from 350 nm to 650 nm.
 16. A displaydevice according to claim 13, wherein each passive compensation retarderof the pair of passive compensation retarders has a retardance for lightof a wavelength of 550 nm in a range from 450 nm to 550 nm.
 17. Adisplay device according to claim 1, wherein the other of the first andsecond retarders further comprises at least one further passivecompensation retarder.
 18. A display device according to claim 17,wherein the at least one passive compensation retarder and the at leastone further passive compensation retarder have retardances that aredifferent.
 19. A display device according to claim 17, wherein one ofthe at least one passive compensation retarders and at least one furtherpassive compensation retarders comprises a passive compensation retarderhaving its optical axis perpendicular to the plane of the passivecompensation retarder and the other of the at least one passivecompensation retarders and at least one further passive compensationretarders comprises a pair of passive compensation retarders which haveoptical axes in the plane of the retarders that are crossed.
 20. Adisplay device according to claim 1, wherein each alignment layer has apretilt having a pretilt direction with a component in the plane of theadjacent liquid crystal layer that is parallel or anti-parallel ororthogonal to the electric vector transmission direction of the displaypolariser.
 21. A display device according to claim 1, wherein theretardance of the at least one passive compensation retarder of the atleast one of the first and second retarders is equal and opposite to theretardance of the respective at least one switchable liquid crystalretarder.
 22. A display device according to claim 1, wherein the firstswitchable liquid crystal retarder further comprises electrodes arrangedto apply a voltage for controlling the layer of liquid crystal materialand the second switchable liquid crystal retarder further compriseselectrodes arranged to apply a voltage for controlling the layer ofliquid crystal material.
 23. A display device according to claim 22,further comprising a control system arranged to control the voltageapplied across the electrodes of the at least one switchable liquidcrystal retarder.
 24. A display device according to claim 23, whereinthe control system is arranged to control the voltage applied across theelectrodes of the first switchable liquid crystal retarder to be commonwith the voltage applied across the electrodes of the second switchableliquid crystal retarder.
 25. A display device according to claim 24,wherein the liquid crystal material of the first switchable liquidcrystal retarder is different from the liquid crystal material of thesecond switchable liquid crystal retarder.
 26. A display deviceaccording to claim 1, wherein the backlight provides a luminance atpolar angles to the normal to the spatial light modulator greater than45 degrees that is at most 33% of the luminance along the normal to thespatial light modulator.
 27. A display device according to claim 1,wherein the backlight provides a luminance at polar angles to the normalto the spatial light modulator greater than 45 degrees that is at most20% of the luminance along the normal to the spatial light modulator.28. A display device according to claim 1, wherein the backlightcomprises: an array of light sources; a directional waveguidecomprising: an input end extending in a lateral direction along a sideof the directional waveguide, the light sources being disposed along theinput end and arranged to input input light into the waveguide; andopposed first and second guide surfaces extending across the directionalwaveguide from the input end for guiding light input at the input endalong the waveguide, the waveguide being arranged to deflect input lightguided through the directional waveguide to exit through the first guidesurface.
 29. A display device according to claim 28, wherein thebacklight further comprises a light turning film and the directionalwaveguide is a collimating waveguide.
 30. A display device according toclaim 29, wherein the collimating waveguide comprises (i) a plurality ofelongate lenticular elements; and (ii) a plurality of inclined lightextraction features, wherein the plurality of elongate lenticularelements and the plurality of inclined light extraction features areoriented to deflect input light guided through the directional waveguideto exit through the first guide surface.